The health pack is a boost that any player can pick up as long as they are lower than full health. Picking up the health pack replenishes your health to full. Health packs are found on every map.
The health pack is most effective when you are on low health so you can build up an even bigger kill streak.
If you don't want others to pick it up, you can throw a gas grenade on it or purposely lose a little health and pick it up.