Multi-user, multiple input, multiple output (MU-MIMO) first appeared in Wi-Fi 5 and allows one access point to communicate with multiple devices simultaneously. Instead of one traffic lane, an access point could transmit data through four separate traffic lanes.
Now, with Wi-Fi 6, access points can deliver data through up to eight traffic lanes to multiple devices, decreasing congestion and increasing efficiency. Specifically, MU-MIMO uses more antennas on the access point to create an equal number of streams for sending and receiving data.
For example, a 2x2 access point has two antennas, a 3x3 has three, a 4x4 has four, and an 8x8 has eight. If you have eight devices, each device can enjoy its own dedicated, uninterrupted stream. Even if you add a ninth device, it will simply share one antenna with one of the previous eight devices. The efficiency will still be greatly improved.
Incidentally, there are some access points that have twelve simultaneous streams, four on the 2.4 GHz spectrum, and eight on the 5 GHz spectrum. It's important to remember, however, that in order to use an 8x8 access point or the 12-stream access point, all devices must be Wi-Fi 6 compatible.
MU-MIMO differs from OFDMA (Orthogonal Frequency Division Multiple Access) in that while MU-MIMO APs provide up to eight channels that correspond to the number of physical AP antennas, OFDMA splits a single channel into multiple sub-channels, which are capable of communicating with several devices simultaneously.