You can perform a number of app functions with Google voice commands, like playing music for example. While Google usually lets you choose which app to use each time, it will now learn which apps you prefer and auto-select them.
As Android Police points out, this doesn't just apply to a single command, but rather a range of commands for a particular type. So, for instance, if you say "play AC/DC", Google will recognise that this is a music-related command. Later, if you say "play music by Queen", Google will remember which app you prefer to use for music commands.