MIMO is just the latest development of a respectably ancient idea -- more wire in the air means more signal. The Yagi, your basic rooftop TV antenna, has a series of parallel elements that act like lenses, focusing the incoming signal and making it stronger. You can get better results still by stacking two or more Yagis, as common sense would suggest. But multiple receiving antennas have an unexpected benefit -- they give you diversity.
One of the constant demons of wireless is multipath, where a signal comes in from a single transmitter from several directions at once. This happens because buildings, hillsides and many other objects either side of the main path between transmitter and receiver can act as reflectors and refractors. The result is like shining a torch through a chandelier. As the many scattered signals take different paths and thus different times to arrive, they turn up with all sorts of different phases to the main signal and can cancel out or boost it seemingly at random. The result at any one point is a mess: that's the reason FM transistor radios can sound so rough at one point in the bathroom yet give perfect results if you just reposition the antenna.
Two or more antennas can easily sort out this mess. At its most basic, the receiver just has to pick the one with the strongest signal at any one time, although there are cleverer ways to analyse the signals and reconstitute the original. This is called diversity reception, and it's been in use since the Second World War: it's also the reason some wireless LAN base stations have two antennas.
For completely different reasons, it's also a good idea to have multiple antennas at the transmitter. By sending signals with slightly different delays to an array of antennas, the resultant transmission can form a tight beam: by changing those delays, the beam can be electrically steered at high speed. Called phased array antennas, these devices are frequently reinvented and given names such as smart antennas, but once again the original ideas can be traced back at least fifty years.
Lately, the idea of using multiple antennas on both transmitter and receiver -- hence Multiple In, Multiple Out -- has come into fashion. This is because while there are some extremely tempting extra benefits to be had by combining the two ideas, they take a lot of signal processing at the receiver to work, and until recently this was too expensive to be worthwhile. Now we have million-transistor chips running at gigahertz and costing thruppence ha'penny, the fiscal equation has changed. The maths of the physics is exciting too -- the potential gain is a simple multiple of the total number of antennas used