Usually, metagamers are considered the elite gamers, who usually have been playing the game for the longest time period, and have the most skill and play the most competitively, the metagame is the environment that these competitive metagamers play in, in any given game, and the strategies used by those top competitive players.
The metagame is usually much more narrow then casual gaming, and there is often very little variation in strategy between metagamers because only the best are used.
Metagame often involves strategies found or created by players, not just simple tactics designed by the game developers to intentionally be part of the game, while also playing by the standards other metagamers believe to be commonplace.
Games such as mario bros or freeze tag usually lack a metagame, because the multiplayer aspect is limited, and/or there is little to no strategy or skill involved in the game, simple games in general usually lack or have a very small metagame, while more complex and competitive games usually have a larger metagame and a larger gap between strong and weak players.
In yugioh, the worlds most popular tcg*, the metagame consists mainly of lightsworn, vayu turbo, gladiator beasts, and several other top tier decks*, players using low tier decks such as Legendary Ocean decks*, usually would lose to the metagame decks within a few turns, due to the abilities, combos, and raw power, of the metagame decks.
Person: "Urgh I hate the meta, there is no originality and I always lose to them, everyone uses the exact same strategy"
In Metroid Prime Hunters, which was considered the best FPS on the ds for a long time, and still is arguably the best*, almost all metagamers would use the imperialist as a weapon, and sylux or trace as a character, most metagamers consider alt forms to be spammy and noobish, because they don't take much skill to use and allow noobs to beat players who do not use alts but have significantly more skill.
*(at the time of this writing)