Definition
This idiom is used to caution against being overly optimistic or making plans based on uncertain future events. It advises against counting on success before it is assured.
They haven't signed the contract yet, so don't count your chickens before they're hatched.
She's signed up for the competition, but hasn't started training, so don't count your chickens before they're hatched.
Before we get the research results, let's not count our chickens before they're hatched by assuming our hypothesis is correct.