It means to avoid worrying about future problems or difficulties before they actually arise.
In other words, it's about focusing on the present and not letting yourself get overwhelmed by things that may or may not happen.
Here are some examples of how it might be used:
* "I'm not going to worry about finding a new job until I lose this one. Don't cross that bridge until you come to it."
* "The project deadline is still months away. Let's focus on getting the current tasks done and don't worry about the bridge until we come to it."
The idiom is a reminder that it's pointless to stress about things that are out of your control, and that you should only deal with problems when they actually present themselves.