A relapse is slipping back into former ways of life, after you have built changes for the better. This could be anything you know you to be wrong or dangerous for you to be doing. A relapse also includes falling back into illegal behaviours.
For example, looking at adult pornography may be legal, but you know this is risky behaviour which feels wrong, and has the potential to push you towards accessing illegal material online.
Relapse prevention is about recognising that difficult times can happen so it is important to be prepared for them.