"Enjoy, bon appetit."
These are conjugations of נהנה in future (which is used here as an imperative): תהנה is second person masculine singular, תהנו is plural, תהני is feminine singular.
When can תהנה be used? Eg when watching a move? Enjoy your movie. Or enjoy yourself.
Is it a food word? Or when else can i use?
It can be used when watching a movie, it's not a food word. It can also mean "have fun".
Why is בתיאבון one word, and not two separate ones? What is the literal translation of that word? Is it a compound word?