In my opinion, a “Bag” means that a player has versatility with their scoring and has a deepened skill set that sets up multiple ways of scoring. Obvious example of an NBA player with a bag is Kyrie. He’s the greatest dribbler ever.
Another less common example, in my opinion, is Reggie Miller. His handle wasn’t elite, but he used his insane jumper to set up the rest of his game. Reggie was elite without the ball and was actually a great finisher (if you look at offensive +/- he’s arguably the greatest playoff scorer ever) because he used the versatility of his jumper to open up driving lanes.
An example of an nba player with a poor bag is Dwight. Now, you might be tempted to say “wait but he’s a great roll man, dunker, and his post game was okay.” And you’d be correct but the problem is that he has zero versatility and lacks a deepened skill set. If you can contain Dwight rolling to the hoop, he’s not scoring (unless he grabs the offensive board after Jameer Nelson bricks the elbow jumper, which he did very often).
I want to add another distinction here just because a player doesn’t have the best bag doesn’t mean they’re not good. Dwight Howard is an incredible center and was a top five player in the NBA at his peak.
Having a bag specifically means having the versatility to score, and having a deep enough skill, set to have options and countermoves. To go back to the Reggie Miller example if Reggie comes off of a screen, and the defender is playing him too tight, he’ll attack of the paint, which he did with quite a lot of success. If Dwight Howard catches the ball on a roll, but he’s not quite in position to dunk or perform an easy lay up, he doesn’t quite have the skills to convert in those situations (although prime, Dwight Howard was considerably faster and more athletic than any other center in the league, so he was usually so fast that he was more than capable of getting into phenomenal position to get the easy lay up)