This idiom is often used in situations where things aren’t going too well, and you need to prioritize what the most urgent matters are.  If someone mentions a problem that can be dealt with or solved later, you use this expression to tell them that.  This is especially true if the problem may never arise.

A: Sir, sales are down and costs are rising.  We may have to raise prices!
B: Things are looking very bad.
C: We may also have to cancel the Christmas Party next year!
B: We’ll cross that bridge when we get there.  First, let’s try to solve our immediate problems.

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