"Meta-gaming" is basically being too smart for the DM's own good.
You know how you go to a stupid movie and watch all the stupid characters doing stupid, irrational, predictable things that stupid characters in a thousand other stupid movies have done, and you think "why don't these guys ever wise up and take the intelligent, efficient course of action?" Well, the DM who accuses you of meta-gaming is usually mad that you're not making things easy for him by behaving like one of those brainless clowns.