What is short squeeze?

It is quite simple. Many many people are short. The price goes up. That can lead to a short squeeze.

How?

People that are short lose money. At some point the broker liquidates their short position - which puts buy pressure at the market (as the broker is buying the outstanding shares at market price). WIth enough people short, this generates enough buying pressure to push the price up, liquidating even more people. The result is an extremely violent upward move as an avalanche of short positions get liquidated.