To make it simple.
It’s basically a shared non canon call of duty universe with characters from the original and new MW games, Black Ops series, Zombies, World at War and Ghosts games. There is like an anti terrorist group that contains members like TF141, MW multiplayer and campaign operators, Black Ops characters trying to stop a terrorist group which has villains like Menendez, Makarov and Rorke from controlling and destroying the world.
The story is still going on and not finished but that’s all you need to understand for now