The BFD Groove engine is BFD3's sequencer or 'virtual drummer': it provides access to a large library of drum performances as well as providing the ability to edit these performances or create your own. When you first launch BFD3, the groove engine will be enabled.
Some users may prefer turning off the groove engine and record their drum parts manually. If this is how you prefer functioning in BFD, you simply will need to press Grv. off on the top right section of the screen, as shown below.
From here, you should then be able to record your drum grooves in real time!