I don't have any Kosher salt on hand and I'm cooking a rib eye steak tonight. Can I just use table salt, or would that drastically effect the taste of the steak?
Why is Kosher salt better to use?
The reason people choose Kosher/Sea/Rock salt over table salt is mainly down to the crystal size and the lack of additives like iodine.
Kosher salt is less soluble and less dense than table salt.
The large crystals in these salts mean that unless there is a fair bit of water present they don't completely dissolve. This means it is less likely you'll over salt steak even if it's caked in crystals.
Table salt will dissolve with far less water present and will, therefore, get absorbed more easily into the meat.
Also the lower density means you can liberality sprinkle Kosher/Sea/Rock salt on things and even if it does all dissolve you've actualy added less salt than you would think. It looks like a lot but 1g of Kosher/Sea/Rock salt takes up a lot more space than 1g of table salt.
1tsp table salt would be way too much. I'd go for about half the volume of Kosher salt if you were dissolving it in liquid and maybe as little as quarter the volume if I was using it as a rub (as in salting a steak).
Kosher salt is processed differently and has no iodine in it, so some people like the flavor better. It's not going to make that much difference, any salt will do. Just don't oversalt it, you want to taste the meat, not the salt.
You don't describe the recipe, so it's hard to say why the author insisted on Kosher salt. If you're supposed to rub the salt on the steak to remove surface blood, then kosher salt is more effective than table salt. If the salt is just a seasoning, Kosher salt (which has bigger granules than table salt) will add a grittiness that some people enjoy.
Either way, you're not going to ruin the steak just by substituting table salt. You just won't get the precise effect the recipe was aiming at. Which is a good thing or a bad thing, depending on the taste of the people eating the steak, but a minor thing in any case.
The difference in salt is more related to how the salt is being used. So for the purpose of quickly salting before cooking or at the table any type of salt will do.
However if you are salting your meat at least an hour in advance and letting it rest before cooking (which I HIGHLY recommend you do) then using the larger grains of Kosher salt has a positive effect. Salting the meat causes the proteins contained in the meat to change and allows the meat to retain juices significantly better. At the same time some of the salt is pulled into the meat as the water that has been drawn out of the meat through osmosis is reabsorbed back into the meat near the end of the hour. Using a finer grain salt, such as table salt, will allow significantly more salt to be absorbed into the meat. Using a coarse grain salt, kosher, will mean a more perfectly salted piece of meat.