To wave at someone means: to raise your hand and move it from side to side as a way of greeting someone, telling someone to do something, or adding emphasis to an expression Hope this helps 🍀

Brad pitt dressed up like a chicken and "waved" at the cars on the road. Here it means that Brad Pitt was waving his hands as a gesture of greeting- like hi, hello 👋

Just wanted to add that wave should be in the past tense in this sentence: "waved at"