I mean what does it do though?
Does it just open up more features for those doing graphic change hacks or sound hacks?
More knowledgeable folks have answered, but the high level version for basic hacking purposes is that a more advanced mapper allows a game to use more memory (for graphics, code, data, sound, etc.) while potentially making other kinds of things possible under the hood. If you think of a processor as a desk, the books you can fit on the desk at any given time represent the amount of memory the chip can work with (call it one page). A game happens one frame at a time, during which the processor has many cycles to compute what should happen in the next frame. A memory mapper allows you to swap other pages of memory in this interstitial time, such that you have the illusion of a large memory. As if you have a vast desk that can hold dozens of books from the library open at once. Even though all those books would never fit on the desk at once, if you swap them out fast enough, it is as if they do.
The MMC5 is a powerhouse, released near the end of the NES run. Sadly, its potential was not realized. In addition to having "a lot" of addressable memory, there are additional sound channels for music and effects, a dedicated multiplication register that yields the answer as fast as you can read it back, 1K of RAM available to you all the time without swapping pages (enough to fit all the custom logic I've written for any NES hack I've worked on)... A marvelous beast. I wish NES developers had continued to work with it at the height of their game, but the market moved on.