It's a Hindi word popularized by Jamie Oliver. It literally means 'cooked', but when used in slang, it gives emphasis. And the British twat pronounces it wrong - you say 'PUK-kah', not 'pukka'.
It can mean everything is okay, finished, complete.
Jim, this is the pukka weed my friend. Where did you find it?
He forgot the ketchup? This MacDo isn't pukka!