You wet them and as the water evaporates, the "towel" cools in temperature.
It's more than just a wet compress. The "towel" is made from polyvinyl alcohol, and the through an "endothermic reaction" it takes in surrounding heat as the energy needed to expel the water...making it cold to the touch until dry...then re-wet and it starts all over.
I used one on our trip in August and it was wonderful! It kept me from melting in the sweltering heat. If you have a ziplock Baggie with you, you can put some ice water in it to moisten the Frogg Togg. It felt amazing in the heat!
I just got one this past christmas and will be bringing it with me this coming August. We were there last year around the same time and it was not horrible but I am sure this would have help out staying cool
You may want to get one and try it out first. I bought one based on recommendations here and did not like it and ended up throwing it out. I have seen other people say the same thing so I know it's not just me!
I also recommend getting one and trying before buying a bunch. Tried one, didn't feel anything more than a wet towel. There was no cooling affect. Have tried 2 or 3 different kinds with same results. I wouldn't waste the money