Jamesy wrote:
"A missed opportunity is a missed opportunity you can't go back."
It is literally what it means. When you have a chance at the very moment you either take it or you miss it.
An example would be that if you work at a company and you see something that would get yourself higher up (AKA on better pay) in the company. There will be a time-frame in which to do this and ignoring it would mean losing the chance to get that higher pay.
Another is you are in a conversation with someone and you see a chance (An "opportunity") to get something in that you want. However leaving it would mean the topic would go too far to mention it later.