Most alcohol prep pad are usually Isopropyl Alcohol. There are used for disinfecting stuff. For example, you would want to use it on your stethoscope before and after using it on someone, or o your skin to clean a spot to insert a needle. It's used for cleaning because it kills bacteria and evaporates quickly.
They are also flammable, so you can use them to help you start a fire. It's also good for cleaning electronics, since it evaporates quickly and leave little to no residue.