It's a list of your stamps, ordered by time of usage. If you delete it, you won't be able to use your stamps until you Rescan in the stamp browser, and even then, the order will be all messed up. In general, TPT expects the contents of the Stamps directory to be consistent with stamps.def. If they're not, there might be perfectly valid stamps inaccessible from the stamp browser, or long gone ones still listed, which when you try to load, you get an error. Tl;dr if you do any manual shuffling of stamps, you have to Rescan to prevent weirdness.