Without more context, the phrase doesn't mean anything other than literally missing a bus (the moving vehicle). But, depending on the context, it could be another way of saying "I missed the boat," which is a idiom for expressing a missed opportunity. "I missed the boat on signing up for the class, and now I can't enroll." Or, "I missed the bus on learning French in school, and now I don't have the time or money to study it." (Although we here at italki know that's there's always a new opportunity to learn a language again : )