Lip licking is an appeasement (if the dog is licking another's lips/muzzle) or self-calming behavior (licking their own). Some dogs are more prone to it than others, like people and thumb sucking or nail biting. Often what starts out as a stress response over time becomes habitual, or even compulsive (again, like thumb sucking or nail biting). So your dog may have once responded to things that made him nervous with lip licking, and over time he's come to do it more absentmindedly. If the behavior seems to be bothering him or causing him distress, like if he gets that kind of panicked "what is my tongue doing? I can't seem to make it stop!" look in his eye, then it's probably a compulsive response, and you can distract him with something to do, like give him a chewy or toy to occupy his mouth so he can't lick. If it doesn't seem to bother him, it's probably just habitual, and I wouldn't worry about stopping him unless you want to.
That said, if this is a new behavior and he's never shown any tendency to lick his lips before, or if you are unable to interrupt the behavior while it's happening, then it could indicate some kind of neurological problem that should be investigated.
This is a cue that he is anxious and or nervous , he may a high energy dog who needs to burn off energy .