There are two types of playlist in the Google Play Music app on Android phones and tablets and there is the standard type, plus one called Instant Mix.
It is an automatically generated playlist and the music is chosen by the Play Music app itself. It has to be said that this is basically Google's version of Apple's Genius feature on iOS.
Start listening to a track and then tap the track at the bottom at the bottom of the screen to go to the currently playing track screen. Press the three-dot menu button at the top and there is an option called Start instant mix. Press it.
What happens is that the Play Music app analyses the track that is playing for its genre, mood, rhythm and so on. It then searches through your music library for music that is similar to it. It then creates a playlist consisting of 25 tracks.
It is great for creating instant playlists based on your current mood and you just have to select one track and let Instant Mix select and queue all the others.
After creating an instant mix or two, you can swipe in from the left and select Instant Mixes in the menu. They are then listed and you can replay any of them simply by tapping them.
There is a menu button under each one that enables you to delete them and there is a button at the top to Create new instant mix.