Meta is weird.
Say you have a game with various weapons. Each weapon has attack power, attack speed, and weight. If everything is balanced right, there could be no singular optimal weapon.
And let's say that the best considered weapons are all of a certain weight. They do the most damage in the least amount of time available.
Playing the metagame would be recognizing the game everyone is playing is using those weapons as best as possible. And then choosing the knife because it lets you run faster than everyone else. And your strategy becomes knifing people in the back and running away before they can fire back.
Then more people catch on to the knife strategy, because the best players are using it. Now most people are using knives and the game is all about maximizing run speed.
So you start using the pistol. It's a very slight hit to your run speed, but you can still fire back on anyone who knifes you in the back. You start just capping knifers left and right.
More people catch on. Pistols beat knives. As more people become pistol wielders, knifers get worse. The game is all about pistols now.
Until someone realizes that shotguns and rifles are only slightly slower than pistols, but do way more damage. Basically, everything of a certain weight is just good when facing a lot of pistols. So the game comes right back around to the start.
This is different from in-game strategy. In-game strategy would be planning routes and knowing how and when to cover. Meta is out-of-game strategy.
Like, investing in infrastructure, technology, etc is all war meta. It lets you approach the situation in ways your opponent isn't thinking about. You can make their strategies during the war irrelevant.