do not cross that bridge until you come to it

Hi! Great question. "Don't cross that bridge until you come to it" is an idiom meaning, "Don't worry about something until it actually happens." Often we say "I'll cross that bridge when I come to it," meaning, again, I'll deal with that when it actually happens. 

For example:

A: I really like Mike, but I'm not sure if he wants to have children. What should I do?

B: You'll cross that bridge when you come to it! After all, you just met him last week.

Hope that helps!

Amy