It's called a bubble when something is hugely overvalued. Usually this happens when lots of people are so excited about something that they put a lot more money into it than it is really worth. At some point, the market realizes that the actual worth of this thing is really a lot lower and people start selling it off. That's when the bubble pops. The financial crises of 2008 was also a bubble, caused by too many people buying into real estate (facilitated by shady practices of banks). Some people say Bitcoin is in a bubble because the excitement over it is huge and a lot of money is flowing into it. Hope that helps.