"What's right isn't always popular. What's popular isn't always right." Albert Einstein

What this quote is saying is that the right thing to do isn't always a common thing to do or is something that is done rarely. What everyone mostly does is more common than what's right to do. For example lets say your cheating on a test, you already know that its wrong but you say to yourself well if everyone else is doing it than why not. That is more popular than what doing the right thing is. If you are not cheating than that is less likely what other people will do.