It is often used in situations where someone has accomplished something impressive, particularly in the context of money, relationships, or career.
Example:
* "I just hit a big one in the lottery!"
* "She's been dating a really successful guy - she hit a big one there!"
* "The company's new product is a huge hit, they really hit a big one."
The phrase "hit a big one" suggests a sudden and unexpected success that brings significant rewards or benefits.