ABI Research has been covering 802.11ac and 802.11ad (WiGig) for about 6 or 7 years now. These standards started to take off commercially a few years ago. 802.11ac is much more complex than previous standards, so it was broken up into two waves of certification by the Wi-Fi Alliance. This made sense to ease the Wi-Fi ecosystem into 802.11ac. 802.11n had introduced the option to use the 5 GHz band in addition to the 2.4 GHz band, and introduced MIMO. MIMO allows Wi-Fi to use the way radio waves naturally bounce around to add more data streams that increase speed and range. 802.11ac only uses the 5 GHz band, and Wave 1 pushed the Wi-Fi ecosystem much faster into the 5 GHz band. We are already past the point where most of the shipping products with Wi-Fi support both bands.
802.11ac Wave 2 extends the highest order of MIMO configuration supported from 4X4 to 8X8 and expands the largest channel size possible. 802.11n in 2.4 GHz supports 20 MHz channels (like older protocols) and in 5 GHz it supports 40 MHz channels. 802.11ac Wave 1 supports 80 MHz channels and Wave 2 supports 160 MHz channels. 802.11ac Wave 2's most important feature is the addition of MU-MIMO. You can have MIMO in the access point, but the transmissions to client deivces work in the configuration those clients have. Most devices are 1X1, and an increasing number are 2X2 and higher. The access point typically works with one client at a time. MU-MIMO allows the access point to split the data streams among multiple MU-MIMO enabled clients. This maximizes the capacity of the Wi-Fi network as a whole and allows the MIMO-enabled access point to put those data streams to much better use.
There is a catch - the clients devices must be MU-MIMO enabled. Non MU-MIMO enabled clients will still work fine, but the access point must communicate with those one at a time. MU-MIMO clients can be grouped together. One way to split Wi-Fi protocols into two groups is into those that are not and are MU-MIMO enabled. 802.11g, 802.11n, and 802.11ac on one side, and 802.11ac Wave 2 and 802.11ax on the other side. In a few years from now, 802.11ax devices will start appearing with uplink MIMO. 802.11ac Wave 2 devices will not slow down an 802.11ax network so much, but those without MU-MIMO will be far less optimized.
Read more about MU-MIMO here in this publicly available white paper: https://www.abiresearch.com/pages/mu-mimo-and-802-11ad/