It's used to increase data rates in radio transmissions. Rough intro: if you have a cabled connection and you want to double your data rate, you can use a second cable. Likewise, if you have a radio transmission between two antennas, you can increase the data rate by adding one more antenna on each side (hence Multiple Input Multiple Output). The signals of both transmitters will arrive overlapped on both receivers, but it is usually possible to detangle them with a system of equations.