What does it mean when a man looks at other women when he is with his girlfriend?

That he thinks the other woman is physically attractive? Looking at other women (just looking/noticing attractiveness) isn't the same as cheating or wanting to cheat, IMO. Noticing attractive people is human nature, and there's nothing wrong with noticing that someone other than your SO is attractive unless you act on that attraction, or are openly staring and obviously checking them out cause that's just rude.