Crouch Jumping in PUBG is a technique that allows you to jump higher than you can regularly. With the added height you can climb objects like crates, leap through windows and jump over low walls.
To crouch jump press 'C' and 'Spacebar' at the same time. That's all there is to it! You can use your thumb to hit both keys or use your thumb and index finger.
If practicing in an area with no windows or walls. You can look in the bottom left corner of the screen. Your icon will show you as crouching when you pull off a Crouch Jump.
You can't jump through any windows with bars over them.