The “hip hinge” movement pattern is your basic ability to move from your hips and not your lower back. It is a vital movement on your mountain bike because without it you simply can not achieve good, balanced body position. Being able to get your butt back and chest down while maintaining a long spine improves your balance, pedaling power and reduces the amount of stress on your lower back and shoulders, however most riders have trouble with their hip hinge and so struggle with applying this concept to the trail.
On page 5 of this PDF handout covering the Functional Movement Screen is the Active Straight Leg Raise screen and if you struggle with it (or more simply struggle to touch your toes) then these exercises here will help you fix the movement issue. Once you have improved your hip hinge you will find yourself able to more easily achieve strong, balanced body position on the bike.