Researchers at Arc Chart wrote a paper offering a general description of MIMO, or multiple input, multiple output: The technology employs multiple antennas at the receiver and the transmitter which can ultimately have the effect of boosting throughput.
Arc Chart positions MIMO in competition to folks like Atheros who have developed their own methods for increasing throughput on Wi-Fi gear. Apparently Intel has said it fully supports MIMO and plans to build the technology into all its future gear, including WiMAX. Intel also wants MIMO included in the 802.11n standard.
For more background on MIMO, check this ZDNet article that ran in October.