I saw this ingredient (farro) listed in a recipe that I was interested in making; however, I've never seen nor heard of it before. Could you explain what it is, please. What can I substitute for Farro?
Farro is a grain, a bit nutty, usually cooked to soft, but with toothsome body.
Here's a good article about it from NPR
Pearled barley would be a good substitute, but if your recipe includes cooking times and instructions, those will vary if you substitute out the grain. Follow the package instructions for whatever grain you do use.
I do prefer barley "pearled", but that means that most of the bran is polished away along with the inedible hull, so it's no longer a whole grain. You can also get it with just the outer hull removed, so that it is still a whole grain. I prefer the texture of the pearled, but either choice would be a perfectly acceptable substitution for farro.
EDIT: I concur with Matt Ball's comment though. If you're in a part of the world where it's easy to get (that includes all of the US), you should try it!
Most farro enthusiasts would say that there is NO substitute for farro. More realistically, you certainly have options. It would help if you clarified what type of recipe you were interested in making - in the absence of that information, I would suggest you consider barley if you're making a soup and quinoa if you're making a salad. The cooking times for both of these would differ from the cooking time for farro.