Memu simulates previous versions of the Android operating system (Lollipop and KitKat, with more on the way). It does this on other, desktop operating systems, meaning you can run apps and games that are exclusive to Android on other devices. You can also tinker with the Android environment itself, get a feel for how it works, or test things as a developer.
Emulated, but not usable
Note that as an Android emulator, Memu is not an appropriate choice for some games. For example, though it can run Pokémon GO (as its own website advertises), a desktop does not have the GPS (or mobility) needed to actually walk around and catch Pokemon. Instead, you'd need to compensate with GPS faking software, which (as it often is) is against the game's rules. With this understanding, though, Memu works perfectly. It simulates the latest versions of Android and, with a computer that can handle the task of emulation, can both play games and run other apps.
For developers and users alike
For some purposes, Memu won't replace a phone. But if all you need is to play a game or there's some specific functionality an Android app has, you can do it from your desktop. It's also an excellent tool for small app developers. If you need to run Android on your desktop, look here.