" Koi Baat Nahi" - Can be used to Convey
It's Fine
For Eg. You Drop coffee on someone's shirt and U say I am so sorry.
To be polite the other person might say "Koi Baat Nahi" meaning "It's fine"
Also Used in a Sense of "Never mind"
Suppose you were unable to clear an exam.
When U apologize for ur inability to pass, your Father might say
He Might say - Chalo Koi Baat Nahi, Agli baar zyada mehnat karna.
(Never mind, Next time work harder)
Hope it Helps :)