Spatial streaming is primarily observed in wireless communications where multiple-input-multiple-output (MIMO) is being used. When wireless signals are being transmitted or received simultaneously in a MIMO environment, the signals being transmitted by the various antennae are multiplexed by using different spaces within the same spectral channel. These spaces are known as spatial streams.
Techopedia explains Spatial Streaming
In a MIMO environment, multiple antennae are used, both at a receiving station and at a transmitting station. In order to transmit and receive various signals simultaneously, a common multiplexing technique known as spatial multiplexing is used. Basically, when wireless signals are being transmitted simultaneously from different antennae, each signal is transmitted via a spatial stream within the given spectral channel to avoid collisions.