"Craic" is an Irish word of broad ideomatic use in Irish English.
On its own, the word means "fun" easy good times, traditionally at a pub in the evening. That's where you go for the craic, traditionally, and that's what you go there to find.
"What's the craic" means "what's up." Ce faci, que pasa. How're you doing. Typical response is "Ah sure, you know y'rself...". It's a simple greeting.
"How was the craic" inquires about a specific occasion.
"Great craic" describes a good occasion.
"The craic was 90" is the superlative.