There is generally one target within the different standards: to attain a higher physical layer (PHY) rate and, therefore, offer a higher speed to users. MIMO and spatial streams form an important factor within this context, but, in addition, the channel width is also always checked.
The simplest way in which to attain this is to increase the channel width. We can double the bandwidth by going from 20 MHz to 40 MHz. This can be compared with an additional lane on a motorway: if there are 4 instead of 2 lanes, double the number of vehicles can use the motorway at the same time.
The disadvantage is that the available spectrum is scarce and, therefore, endless expansions are not possible. This is what we are now noticing with the 802.11ac standard where 80 MHz channels are possible. We have 19 channels in Europe if we use 20 MHz channels. This becomes 9 when we switch to 40 MHz (11n) where 5 remain if they are 80 MHz channels. Wave 2 brings 160 MHz channels of which only 2 are available. At least 3 non-overlapping channels are required to set up a professional network and, if you also want capacity and minimum co-channel interference, at least 4 or 5.
In addition, there is a large section of the spectrum where dynamic frequency selection (DFS) must be used. This means that these channels cannot be deployed reliably for critical networks. This means that the number of available channels at 20 MHz is 4, at 40 MHz is 2 and at 80 MHz is 1.
MIMO is the acronym for Multiple In, Multiple Out. This means that the radio is split up in multiple radio chains that can all transmit and receive data individually and simultaneously. More data can, therefore, be sent or received simultaneously or the same data can be sent or received several times for a reliable signal.