Hi dreemwest,
A radio system sends multiple radio signals at the same time. Each of these signals is called a spatial stream. A spatial stream is sent from its own antenna using its own transmitter. Each spatial stream follows a different path to the receiver. The radio system can send and receive signals in different spatial locations.
More information please refer to https://forum.huawei.com/enterprise/en/From-Beginner-to-Expert-WLAN-Common-Terms-Section-17-Spatial-Stream/thread/532795-869
I hope it helps!