MIMO (Multiple Input Multiple Output) is a hardware and software feature enabling the transmission or receptions of WiFi signals using multiple antennas, simultaneously. The main benefit is multiplication of the achieved transmission speed. An additional benefit is longer range.
The number of transmitting and receiving antennas is written as NxM:
It's important to know that even if your home WiFi supports 4x4 MIMO, for example, your client (smartphone or laptop) may support only 3x3 MIMO or even 2x2 MIMO, thus limiting actual achievable speeds.
Most modern WiFi routers employ MIMO. Here are some noteworthy examples that support 4x4 MIMO:
MU-MIMO is an advanced version of MIMO.