It means nothing but "the game in the game" and pretty much every single multiplayer game in existance has some kind of metagame. It's basically a synonym for "strategy".
Examples of metagaming in some popular multiplayer games:
Dota2/LoL: Which heroes do you pick to counter the other team. Which items do you purchase, which order and when do you purchase them? Which ability combination is good for defeating the other players?
Counter-Strike: Which player do you send where. Which positions do you hold. How do you manage your money per round. Where do you throw your grenades. Which strategy do you use to assault a map location. How do you counter the strategy of the other team?
StarCraft: Which build order do you use to respond to what you know about the other player. Which composition of units do you use, which combination works for this situation and this particular map. Which strategy can you use today because tomorrow everyone will know how to defeat it?
Quake: Which route do you go down when you spot your opponent. Which items do you try and pick up and which items do you deny when they re-spawn. Which weapon do you use at this very moment and why?
ED will have metagaming too. Someone will come up with a maneuver that gives them an upper hand in combat. Then it will be popularized and someone will come up with a counter maneuver. Congratulations... metagaming.
My definition of metagaming include elements that goes beyond the physical representation of the gameworld. It is in the word itself. "Beyond" the game. To say metagaming is synonymous with strategy, is to narrow one single aspect down, and quantify it based on a singular aspect in a sea of possibilities.
Real world elements that takes you out of the game setting, will also take you "beyond" the game. Strategy is neither here or there. It belongs in the game, as well as out of game. It is what you base it on, that creates the definition, external sources might not go "beyond" the scope of the nature of your game(world), but they can go beyond the scope of what the game intended mechanically or naturally. Suddenly two different aspects emerge, based on the same origin! So with all due respect, metagaming is *not* synonymous with strategy, for that is a more ageless and encompassing aspect, influenced by the flow of information, more than anything. Information flow is not a metagame concept by itself. It depends on the variables involved.
In short, metagaming represent elements that take you beyond the format of the game, but exchange of ideas is not technically outside of the scope of the game. For we simulate what we already are, so technically you might supplement what is given, or exploit the intention of what is given. Both synonymous with strategy. But it doesn't mean the game should come with support reflecting any of those strategies. That is what I mean by (not) including metagame elements in the game. If it clearly does not belong there, it should reside in the external format. To exploit or expand on. That is half of the 4X right there. Strategy for ya!