Android Go is a version of standard Android OS. For example, there is available regular Android 10 and also Android 10 Go edition. It was built mainly for cheaper smartphones and tablets with 2 GB of RAM or less. The advantage of Android Go is, that it is running lighter and reduces data usage, which allows to device to perform faster.
The impact of Android Go on speed is approximate 15 %.
Android Go also has its own versions of applications, like Google Go, Gmail Go, YouTube Go, Google Maps Go, Google Assistant Go, Files Go etc. These apps may have some limitations in comparison to regular Android applications.
New devices with 2 GB of RAM or less, will have to use Android Go, because new versions of regular Android will have higher hardware requirements.