The Serious Pack is red Backpack that's filled with the maximum amount of ammunition for every gun that the player has at the moment.
In Serious Sam 1 and Serious Sam HD, Serious Packs often appear during or before huge fights against hordes of monsters. This lets the idea stock up on ammo before or during a huge fight and lets them know that a big fight is coming. In Serious Sam: Double D, the Serious Pack is used to let the player restock on ammo during the second to last level's large wave of enemies at the end and the final boss.
If a Serious Pack is found normally, you should be prepared for a big fight as soon as you pick it up, as hordes of enemies will begin to appear soon afterwards.
If possible, try to conserve a Serious Pack until it's really needed. It gives the player a lot of ammo, but it would be a waste picking it up if the player is close to the maximum amount of ammo for their guns. Note that this should be ignored if a Serious Pack spawns right before or between waves of enemies, as picking it up is required to trigger more enemies to spawn.
A handful of Serious Packs can be found in secrets throughout the games it appears in.