The Serious Bomb will take a a few seconds to warm up, then will explode, killing every enemy within a 750 metre (in-game) radius of the player. The Serious Bomb does not harm bosses.
How the player uses it depends on the game; in The Second Encounter, the player simply needs to hit the “0” key to start the timer. In the Xbox version of Serious Sam 1 and Serious Sam 2, the player must take out the Serious Bomb, then hit the fire key in order to start the countdown.
Ideally the Serious Bomb should only be used if the player is being absolutely overwhelmed by enemies and has too little health/ammo to counter them without reloading saves many times. However, experienced players may use it to quickly kill any group of enemies even when not really necessary, if their Serious Bomb capacity is full and they know there's another Serious Bomb coming up soon - letting it just go would be a big loss because of how rare they are.
The Serious Bomb is also useful against hordes of large enemies, such as Large Spiders. This makes hordes of strong enemies extremely easy to deal with, which is very useful no matter how much health/armor/ammo the player has.
In the Xbox/SS2 version of the Serious Bomb, the player should make sure they're in a somewhat safe position before they take out the Serious Bomb, as they'll be vulnerable for several seconds while the bomb's countdown animation is active.
In tremendously wide areas, the Serious Bomb may actually not be able to kill every single enemy in the area. One example is The Pit boss fight against Kukulkan, where depending on the player's position, the Fiendian Reptiloid Demons perched atop corners of the surrounding walls may not be killed by the bomb if they're too far away.