Traditional backend services provide specific solutions as complete units and thus any extensibility or customizability is very limited. Unisave tackles the problem from the other end, giving you a platform to build on. But you as a game developer are solving specific problems and you don't want to waste time coding something that has been done many times before.
Modules are modular pieces of Unisave code that solve these specific game-development problems. You can include only those you need. You can also modify them, as they are built on top of the Unisave platform, just like your backend code is.