A multiple-input multiple-output (MIMO) system is an antenna system that consists of M transmit antennas and N receive antennas. MIMO technology makes spaces the resources used to improve performance and increases the coverage range of a wireless system.
Multi-user MIMO (MU-MIMO) technology is used to increase the wireless rates of STAs. It enables parallel spatial streams occupying the same time-frequency resources to be sent to different STAs. Due to the long distance between users, parallel spatial stream transmission is more likely to be used in an MU-MIMO system to improve spectrum usage and STA rates.
More information:
https://forum.huawei.com/enterprise/en/From-Beginner-to-Expert-WLAN-Common-Terms-Section-18-MIMO-MU-MIMO/thread/532799-869