MicroG is an open-source implementation of Google Play Services. It acts as a "shim," tricking apps into thinking Google Play Services is installed, but it routes the data elsewhere or keeps it local.
If you install a custom ROM (like LineageOS) without GApps, you will quickly notice that many apps will not function correctly. This is because many third-party apps rely on Google Play Services . MicroG is an open-source implementation of Google Play