I consider a pro to be considerably better than an open class player. A pro can make a living playing pool (in tournaments, not just hustling) and is a threat to win or finish well in any event they enter. An open player is usually a very good regional player and will occasionally beat a pro but doesn't have near the consistency to beat pro after pro, match after match to win a major event.
Often you'll see an open player play like a pro in a local event against C+ to A+ players and they'll seem really good but put them up against Archer and they'll fold, sometimes embarrasingly so.