MIMO means Multiple. In practical terms it means multiple aerials sending and receiving signals. The spacing and configuration of those aerials alter the affect of MIMO. You want aerials spread far enough to provide independent paths from your computer to the router providing Internet access.
Think of your wireless notebook in one room trying to connect to a wireless router in another room. Now place a steel column in between the two. Conventional wireless would not work because the steel column would kill the transmission of a single aerial. Substitute multiple aerials and you have a situation where one of the aerials can possibly communicate despite the steel column.
If the obstructions are thin, you can build MIMO as several devices along the back of a router or similar devices. When the obstructions are bad, you need several aerials separated by long cables and it will only work if the controlling chip understands the separation.
Not all situations are solved by MIMO because the MIMO aerials on a typical MIMO device are not that far apart. You can add external aerials to MIMO devices and expand the spacing of the aerials. MIMO only has to be a little bit bigger than an obstruction to work. The obstruction might be a ten metre truck and be beyond what you can do with fancy aerials.
MIMO also assumes small or non existent reflections. MIMO does not work when there are metal walls reflecting the radio signals all over the place. MIMO looks at the strongest signals. When the walls are metal, you can have several signals if similar strength.
While MIMO is not the perfect answer, MIMO does cure a bunch of common problems and is worth the investment.