Boosters permanently improve your stats, just walk over one when you see it and the bonus will be applied. They come in three flavors, as there are three stats: Health, Mana, and Damage.
- Mana Booster: Adds +1 to Max Mana. Mana does not improve naturally as you level, and so Mana Boosters are one-time effects.
- Damage Booster: Adds +10 percentage points to Bonus Damage, which modifies Base Damage (along with buffs) to produce the Damage stat. Bonus Damage is cumulative and recalculated whenever the Base changes.
- Health Booster: Adds +10 percentage points to Bonus HP, which modifies Base HP (again, along with buffs) to find your Max HP. Bonus HP is cumulative and recalculated whenever the Base changes.
Note that Health and Damage Boosters are bonus percentage points. If you had +10% Bonus Damage before, you'd have +20% Bonus Damage after picking up a new Damage Booster; your Damage would then be recalculated off your Base. The same process happens whenever your Base changes (such as level-up). To be clear: it's not +(10% of current Bonus Damage) — that'd be +11% Bonus, not 20%, which is lame. Nor is it +(10% of current Damage) — which would be nearly nothing at level 1, and truly absurd late-game.
You can see this in the UI for Damage: hover over the Damage stat and a tooltip will list your base Damage and a bonus percent. Damage Boosters add to that bonus (+10%); each level you gain adds to your base. Some basic math (applying buffs last) gets your current Damage stat. HP works the same way, but unfortunately a breakdown is not provided in the UI. You'll just have to trust the magic machine's math. Again, the important take-away is that there is usually no benefit to holding off on collecting Boosters, unlike with Glyphs and Items (which take up space) or God perks (which usually depend on your current state).