Is it a high hydration rate? Minimal kneading? High-protein flour to hold hydration?
There are lots of theories about this topic. Basically, to get large holes, you want a dough that has enough air to expand greatly, enough structure to support that expansion, and somewhat uneven distribution of the air/structure to produce various size holes.
Some things that will help:
Relatively high hydration. It's not strictly necessary, but it allows more random gluten connections to be made, making for a more chaotic internal structure. Hydration needs to be compared to flour gluten content. High gluten is not generally necessary, but a good bread flour will help. Nevertheless, it is possible to produce large holes in short or squat loaves (baguettes, ciabatta, focaccia) even with AP flour. The high protein is usually only necessary to get a high-rising loaf. Lower protein flour loaves may still have relatively big holes, but they are more likely to spread rather than rise high when baked free-form.
Gentle kneading at the outset. You really only need to mix until ingredients are thoroughly combined and distributed. Extended kneading at that point is not only unnecessary, but it can encourage the initial gluten connections to be really even (and not the random network of strands and holes).
Instead of kneading at the outset, do stretch-and-fold maneuvers periodically during the bulk rise. They will add a lot of strength while minimally disturbing the emerging gluten networks. And they will encourage more random connections between each stretch-and-fold maneuver.
Don't skimp on the bulk rise time. This is when the big air bubbles and random gluten networks form. Many people even do this overnight in the fridge or follow the "no knead" bread recipes that just let it sit out for many hours.
Don't "punch down" the dough after the first rise. Instead, your goal is to shape the bread as efficiently as possible without destroying the random network of holes that is present. Contrary to popular belief, you don't necessarily need to be very gentle with the dough at this phase (though with some types of bread, like ciabatta, it can help). It doesn't matter so much if you lose some air, as long as you are stretching the gluten to add structure. So, don't deliberately try to get air out, but instead focus on stretching and folding over to get a taut, elastic dough in preshaping. Or, be gentle -- the dough may not rise quite as high, but it should still have large holes.
If you do lose some air during shaping, don't skip a bench rest. A bench rest essentially allows another "stretch-and-fold" exercise that adds strength. Preshape the dough until you can feel it become a bit taut. Then cover the dough and wait 10-15 minutes. Then do the final shaping of the dough, being sure that the surface is taut, which will allow the strongest final rise and increase oven spring. Again, as long as your yeast is strong, the big holes will return even if you lose a little air here. In most cases, being gentle and "trying not to disturb the bubbles" will actually result in less of a final rise and therefore smaller bubbles in the final loaf.
Use any techniques that will increase oven spring. Anything that gets you a bigger loaf means more room for the bubbles to get bigger. That includes: steaming the oven, baking on a preheated stone, slashing the loaves properly right before the bake, baking on high heat (at least for the first 10 minutes or so), etc.
There is no short or easy answer to this. I spent around 15 years learning to master this. In short, the main factors are:
- The right flour and balance between water and flour - depends greatly on flour quality.
- The right kneading - enough to make the gluten into an elastic structure with long threads, but not too much as it will break the structure.
- The right handling of the wet dough - avoiding collapsing the gluten structure.
- The right baking - 25-30 minutes at 225-250 degrees on a baking stone.
You can read a detailed description of my efforts here (including pictures and videos): http://www.rkursem.com/food/bread/fluffy-bread/
IT's sad to see that most answers are wrong. In the past, crops such as einkorn, spelt, kamut, all raised throught long fermentation, so with a sourdough, couldn't give big holes in the dough. Now we have big holes in the dough because recent hybrids of wheat ( that came largely after WW2) have much more gluten and the gluten is not degraded by long fermentation. Instead bakers use very short and powerful fermentation with yeast. That's what makes big holes in bread. So basically big holes in bread is the trademark of lousy bread that is high in gluten, which wheat comes from a modern hybrid and that went through a fast and powerful fermentation that produced a huge amount of CO2, not air, as mentioned in the first answer. It's not air, it's CO2 from the metabolism of the yeast that produces the bubbles that get trapped into very dense and strong gluten. It's amazing how everthing has been understood backwards. Today people believe that good bread must have big holes in the dough (produced by big bubbles of CO2 through fast powerful yeast fermentation) and that a tighter, more even dough is the product of a novice. Wrong! That's the opposite. Crooks masquerating as bakers have become masters at baking white flour inflated with yeast into something that looks rustic. Understand that to call a bread Whole Wheat it must only contain 17% of whole wheat flour. The rest can be disgusting, refined, constipating white extremely refined flour. The sad truth is that most people calling themselves artisan bread makers are cheaters, at best. So to answer your question, use white flour from modern hybrids and use yeast and you'll have big holes in your bread.