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.
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.