This refers primarily to enemies who have a weak point on their back; often in the form of a backpack, usually containing ammo or fuel for a heavy weapon. Often attacking it will result in a large explosion killing both the larger enemy and the enemies around it.
While the backpack is usually fuel for a fire based weapon that does not always need to be the case. In Killzone: Liberation there were enemies that had radio transmitters on their back. Shooting it would short the transmitter and electrocute the unit. Failure to do so stealthily results in the radio being used to call in back up.
Another example of shooting the backpack would be the "Flame Boomer" enemies that made their first appearance in Gears of War 2. This is a classic example of a large enemy with a fuel container on it's back which in this case is used to fuel the scorcher weapon. They are a quite large enemy, so they will take a lot of fire from the Lancer shooting him normally but pop a few bullets in his backpack to set him on fire.
Not all enemies with explosive backpacks are terribly threatening in their own right, though. An example would be the Grunts in Halo: Reach. While a simple shot to the face with a precision weapon will put them down, shooting a few more bullets into the methane tanks strapped to their backs will cause them to meet a more explosive end, possibly damaging the surrounding enemies, as they almost always travel in packs. This is also effective when the Grunts are making a "tactical retreat" and running scared, making a headshot impossible to line up. Note, however, that in previous Halo games, Grunt backpacks were either purely decorative, or would simply break off when damaged, without killing the Grunt. The more spectacular end in Reach was inspired by Halo Wars, in which killed Grunts tend to spiral off into the air before exploding, though these explosions were harmless to other units or buildings.