Metagaming is using knowledge that your character wouldn't have access to, or wouldn't otherwise know. An example of metagaming might be sending a tell to someone when you die to ask them to come and raise your character, when they would otherwise have no knowledge of this. Another example is giving your character an Intelligence score of, say, 10, but allow them to figure out difficult riddles and puzzles with great ease. Even if you know the answer, your character might not.
Yet another example would be "staking out" an un-pickable locked door until it opens and then entering while the other player is in a mechanical area transition delay so that you avoid any RP conflict with the character who had a key (at least RP over-powering them so the door doesn't close on you or enter after they transition out and carelessly neglect to close it).