To deviate from the original topic of conversation, to go off topic. Typically one says this phrase when they realized they have gone off topic and decide to return to the original point.
Another definition is to return to an original point.
Man1: Yes yes I know I'm a slacker and an asshole, but I digress. We need to come up with a plan now. (bad example sorry)
George was doing so well with getting his grades up but he's digressed into that bad behavior of drinking and partying all night and not doing any work.