Select and hold CMD and tap the TAB bar.

This will show a list of open active apps.
Tap the TAB bar to move through the list,

release the CMD key when you get to the app you want.

If you want to close an app keep the CMD key held  down and tap the Q key to Quit the highlighted app.

Who does this?

When you have a lot of active apps, all open on a single screen or desktop sometimes it can be hard to find the one you need. Fast switching can help here.