Multiple-Input Multiple-Output (MIMO) technology is a wireless technology that uses multiple transmitters and receivers to transfer more data at the same time as opposed to the Single-input single-output (SISO) technology, which is a conventional radio system where neither the transmitter nor receiver has multiple antenna.
802.11n is the first standard in the 802.11 family to support MIMO (pronounced "MY-moh").
For optimal performance and range, both the wireless client and the Access Point must support MIMO.
MIMO technology takes advantage of a natural radio-wave phenomenon called multipath. With multipath, transmitted information bounces off walls, ceilings, and other objects, reaching the receiving antenna multiple times via different angles and at slightly different times.
In the past, multipath caused interference and slowed down wireless signals. MIMO technology takes advantage of multipath behavior by using multiple, smart transmitters and receivers with an added spatial dimension, to dramatically increase performance and range.
More antennas usually equate to higher speeds. A wireless adapter with three antennas can have a speed of 600Mbps while an adapter with two antennas has a speed of 300Mbps.