Heist is an objective based game mode.  This game mode is played with two teams of three. Each team spawns on the opposite sides of the map but have different objectives.  One team has to defend a safe containing crystals.  The other team has to break open the crystal containing safe in 2 minutes and 30 seconds.  Sometimes heist will have a bomb in the very beginning that must be destroyed in order to walk easily to the chest.  This is basically to slow down attackers a few seconds since that map is smaller than the others.  The attacking team will have more success with long range or rushing brawlers like Colt, Bull and Barley, while the defending team will have better luck with objective holding champions like Colt, Jessie, and Ricochet.  The maps are never symmetrical as one team has a safe and the other does not.