Reaction is a Game Type in Battery Mode. Reaction is based around Attack and Defend type maps, with one team attacking and the other defending.
In Reaction, the objective of the attacking team is to start a meltdown in the enemy's reactor. The objective of the defending team is, of course, to stop them.
Secondary objectives in Reaction allow the attacking team to meltdown the reactor more easily or to attack it from a different direction, or allow the defending team to close off corridors to the attacking team and make melting down the reactor more difficult.