Monty Hall problem
 

This is probability puzzle loosely based on the television game show Let's Make a Deal and named after its original host, Monty Hall.

Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what's behind the doors, opens another door, say No. 3, which has a goat. He then says to you, "Do you want to pick door No. 2?" Is it to your advantage to switch your choice?
Think about it--
The answer is here: https://en.wikipedia.org/wiki/Monty_Hall_problem

Good Luck
PS I didn't get it