MiMo is an antenna architecture that does semi-magical stuff by intelligent use of two or more physical antennas at one or both sides of a wireless link. The basic idea is that if you have two antennas on each end, you have FOUR potential signal paths among them. This is a big deal because one of those paths is often much better than the others (due to various kinds of interference caused by reflection interference and other phenomena). The radios constantly monitor all possible paths, and rapidly switch to whatever is best at any given instant. It works really, really well.
MiMo can be used in many RF applications (it is why most high-end WiFi base stations have two antennas these days). The particular application we are talking about here involves using it for cell signals. Most modern cell towers are MiMo equipped. I don't know if cell-phones do it, but you can get MiFi hot spots that have dual antenna ports. if you put a MiMo antenna on your roof and plug in both leads to the two antenna ports, you will be a happy person.
The problem with cell repeaters is that, although they do indeed amplify the signal, they also amplify the noise; and it is the signal-to-noise ratio that matters. Just looking at the bars will make you think it is working well, but often (but not always), putting your phone into maintenance mode where you can see the S/N ratio, will show a much less dramatic improvement. Most systematic measurements that I have seen have confirmed that MiMo significantly outperforms repeaters, and it is much simpler and cheaper.
