Invasion is a Game Type in Battery Mode.
In Invasion, there is an attacking team and a defending team. The attacking team deploys from any number of attacking methods, including parachuting out of planes, assaulting out of landing craft, or many other methods. The attacking team is at a disadvantage in terms of support until they capture a certain amount of objectives, which allow heavy support to be brought in. The objective of the attacking team is to capture every objective, while the defending team must annihilate the attacking team and prevent their reinforcement.
Secondary objectives for the attacking team in Invasion are usually used to unlock support early on in the game, and often include capturing enemy equipment. Of course, the defending team can just capture it right back, and so it must be held securely in order to be used effectively. Secondary objectives for the defense usually involve ways to disrupt the influx of reinforcements via the destruction of landing craft or shooting down planes.