This idiom is used to tell someone not to worry about things that may or may not happen in the future. Very often, when we come up with what we think is a good proposal, the prophets of doom in the office give us a detailed account of the things that could possibly go wrong. They take great delight in telling us why the project will fail, and they come up with all kinds of imaginary problems. In such a context, if you tell your detractors that you will cross the bridge when you come to it, what you mean is that you will deal with the various problems as and when they arise. You are not going to worry about them unnecessarily right now.
Examples: What if we run out of funding? We’ll cross the bridge when we come to it. *Mohan seemed unconcerned. He said that he would cross the bridge when he came to it.