An "affirmative tú command" means you are telling someone to do something informally. This could be telling your little brother to eat his meal, your friend to walk with you, and many more scenarios.
When conjugating verbs into an affirmative tú command, when they end in an "ar" ending, we drop the "ar" ending and then add an "a" ending.
Cocinar -> Cocina, it means "Cook!"
Dar -> Da, it means "Give!"