In the Marvel Universe, Venom is a character who gained his powers by being bitten by an alien spider.
In this situation, the alien symbiote that covers Eddie Brock (aka Venom) has bonded with his DNA to create a unique and powerful symbiotic life form.
When Venom is high on adrenaline, he can transform into an even bigger and stronger version of himself, dubbed "Big Venom".
This transformation is impressive and gives Venom unparalleled power in his fights against his enemies.