Mod is the short for Modification. In this case, we are talking about singleplayer or campaign modifications. There are only 2 official SP's (the normal game Populous: The Beginning, and the expansion Undiscovered Worlds). Fan players used programs like Populous World Editor and others to create new levels, textures, scripts and other modifications, added rows of 10 or 25 levels together to create their own campaigns. Most of them are slightly harder than the Bullfrog's original game, but very fun to play. If you are looking for a challenge, you should try a Populous mod. There are many sites where you can find this mods, and you can play them using the ModManager or simply by downloading and replacing files.