When game information outside of what is available in a game is used to give a player an advantage in-game. Most commonly seen and frowned upon in many forms of role playing especially when consent has not been given.
A dungeons and dragons adventure team comes across an unknown monster in-game. The metagamer knows this monster through out-of-game information. Using this information, the metagamer's character acts upon the monster's weaknesses and attributes despite the character having never seen the monster before.
In an online pan-fandom RPG, Ryoga from Ranma 1/2 meets Ken from Street Fighter for the first time and they have a fight to the death. Ken finds some water in a random nearby bucket and dumps it on Ryoga, then proceeds to pummel the life out of Ryoga who is now transformed into a tiny black pig because of his water curse.