Crouching is a stance normally used for rifle shooting. Crouching is a way for a player to have better accuracy and is a preferable firing stance as compared to simply standing up and firing, as well as making one a lower-profile target. However, movement speed and reaction time are lowered due to the increased duration of which it takes to turn around or escape from a firefight.
Crouching is available in every installment of the Battlefield Series. Crouching in game slows movement slightly, and generally increases accuracy. Sensitivity is unchanged while crouching. It is most widely used to take cover and provide the enemy with a harder target.
In Battlefield 1, players can now do a slide-to-crouch maneuver by pressing the button while sprinting. This allows for the player to slide a meter or so before crouching instead of causing them to lose momentum from stopping to crouch straight away.