What is the meaning of jumping on the bandwagon?

The phrase "jumping on the bandwagon" refers to the act of one following a trend, having an opinion, or performing an action that has recently become popular among many people.
It means to follow the crowd example (if you like the sixers and switch over to the bostn celtics because they won the championship) that's a bandwagon
Usually means going with the flow of things. Another meaning can be that you are agreeing with an idea and trying to helpor to copy other people are doing because it is fashionable or successful
If someone "jumps on the bandwagon" then they engage in something that a lot of people have been ingaging in, or liking something/someone that a lot of other people have started to like. For example, if somebody becomes a Justin Bieber fan, they would be jumping on the Justin Bieber bandwagon.

This means to do what everyone else is doing for no other reason than BECAUSE everyone else is doing it.