Drop-in Drop-out means that you will be able to join and leave a match anytime you want. You don’t need to wait to jump into a match, and there is no penalty if you need to leave the match. This functionality also allows you to join your friend’s ongoing match, you just need to tap the Drop-In button after tapping on your friend’s name.
While there is no penalty for leaving a match, you will get more rewards if you play until the end of the match, and more rewards if you win the match.