I’d take a nerf bullet for you.
It is a play on the idiom “I’d take a bullet for you.” The idiom is used to say to a person you like him so much, you’d jumped in front of a gun being fired to save them. Nerf bullets are soft bullets for Nefr branded toy guns. Put together this quote is a fun way to tell someone you like them, but not that much you’d take the real bullets for them. It is used as a funny exchange between friends.