So if you ever read any product spec of wireless access points you will see 2x2,3x3 all the way up to 8x8. But what exactly does this notation mean.
The first number represents how many transmit antennas the access point the second is how many receive antennas and the : and 3rd number represents how many spatial streams the access point has. The number of independent wireless data transmissions that are sent over the antennas on the same channel
These spatial streams are fundamental to MIMO (multiple input, multiple output) antenna technology. Which takes multiple copies of the same data signal and transmits them through physically distinct paths to a receiver which recombines the signal from the various streams.
This notation can sometimes be confused with how many radios an access point has but this notation is describing the spec of the radios not how many radios the access point has.
So what the difference
So the more antenna an AP has the more spatial streams the access point can handle at the same time therefore the more spatial streams allow the access point to deliver more data over the air at the same time, thus increase the bandwidth (data rate) the access point can deliver.
So as you can see from the image above. depending on the spec of the radios in the client device you can get more spatial streams to the client device which means have throughput. with the latest Wi-Fi6 standards you can achieve 1.2Gb (2x600Mb) on a 2x2:2 access point on 5Ghz radio compared to 1.8Gb (3x 600Mb) on a 3x3:3 radio.
The increase in the number of antennas helps improve signal link quality and reliability through a technique called spatial diversity. Because the antennas are physically separated from each other, each antenna sees a slightly different copy of the signal. The signal seen by one antenna may be slightly imperfect in one area of the transmission while the other antenna may see it perfectly. This is especially significant in environments where the RF environment is less than ideal due to site geometry, building materials, and interference.
Also with an access point having a higher number of spatial streams, the AP can communicate with more devices at the same time meaning it improve performance in highly congested areas. with the ability to handle more clients request at the same time