When someone says "Don't bank on it," they are expressing doubt or skepticism about a particular event or outcome. It's a way of warning you not to rely too heavily or place too much confidence in something because the person believes it is unlikely to happen or be successful.
In essence, they are cautioning you not to depend on the thing you are hoping for or expecting because the chances of it coming true or being realized are slim. It's a way of managing expectations and advising against counting on a specific result.