Bloodrush is an activated ability you use from your hand by discarding the creature card and paying its bloodrush cost. Doing so gives a target attacking creature a power/toughness boost and sometimes keywords (like trample or first strike) until end of turn. The creature card never enters the battlefield—it's a one-shot combat trick straight from hand.
The most common mistake: bloodrush only targets attacking creatures, so you can't use it on defense or on a creature that isn't attacking. It also doesn't trigger anything that cares about casting a spell, since bloodrush is an activated ability, not a spell—it won't bait counterspells and dodges effects that respond to spells.
In Commander, bloodrush adds reach and flexibility: those creatures double as bodies or surprise combat blowouts, letting aggressive Gruul-style decks push damage or punch through blocks unexpectedly.