The video can be found here and explains everything, but let me explain it again for completeness.
The Monty Hall Problem is a classic apparent paradox in probability, named after gameshow host Monty Hall from Let's Make a Deal. In the show, the contestants are shown three doors and told behind one of the doors is a brand new car. Behind the other two doors are "worthless" prizes; anything works, but traditionally the problem says the other two doors hold goats. The player gets to pick any of the three doors, and whatever is behind the door is what they win. If they pick right they get a car, otherwise they get a goat.
To add tension, after the contestant picks, Monty Hall would walk to another door, a door that the player did not pick, and show them what was behind it. And look! It's a goat! The car is still out there!
In the Monty Hall Problem (not necessarily the show), Monty then asks if the contestant would like to change their mind.
The question is, what is the probability of the player guessing correctly if they swap their pick?