pH is, put simply, the measure of how acid or basic the water is. 7 is completely neutral. Below 7 grows towards acid, above 7 is basic. Usually--and I say usually because it's not always the case--the more basic water is harder and has more salts and minerals in it, therefore more calcium for the snails to use, and the more acid, the softer the water is and the less salts and minerals are in the water, therefore less for the snail to use to replenish their shell. If it gets too acidic, I've read that it can start almost eating at the shell, but I'm not sure most tap water would get low enough. If you're below 6, you might have issues, but you'll probably have issues anyways since it's harder to keep pH stable that low.
If you want more detailed info, wikipedia's got a good article that gets more into the science of it--I was serious when I said that was put simply, but that's about what needs known for the tanks and my memory for details is not that great.
My betta has a pet mystery snail that he leaves alone, and I have 2 in the 55 gallon.
While it probably won't be an issue, I'll toss it out there just for the edification of anyone reading who's interested and curious--sometimes bettas will start nipping at the snails' antennae. If you see that, pull the snail out. While they can regrow the antennae, if it gets bad enough, the stress can kill the snail.