MIMO (Multiple Users - Multiple Input Multiple Output) is an advanced variant of WiFi MIMO that allows using different antennas to simultaneously transmit to different clients using different antennas. The main benefit is that more clients can get their data quicker, thus achieving higher overall capacity and average speeds for networks with multiple clients.
Some newer WiFi routers already support MU-MIMO. Here are some noteworthy examples that support 4x4 MU-MIMO:
It's important to know that clients (smartphones, laptops, etc.) must also support MU-MIMO for your home network to take advantage of it.