What is oui in french and English?

As a question, i.e. "Ça va?", it means "How are you?"

As an answer, i.e. "Ça va!", it means "I'm OK" or "Fine, thanks". You can also say "Ça va bien, merci" which means "I'm very good, thanks". Or "Ça va mal" which means "Not very good".