Flexion closes the angle at a joint and extension opens it, so a biceps curl flexes the elbow on the way up and extends it on the way down.
They are the two directions of almost every sagittal-plane movement. Knee flexion is a leg curl, knee extension is a leg extension. Hip flexion is bringing the thigh toward the chest, hip extension is driving it back, which is the top half of every squat, deadlift and hip thrust.
The spine is the one where the words matter most. Lumbar flexion under load is the rounding you coach out of a deadlift; thoracic extension is what you coach into an overhead press.
Naming the direction fixes cueing. "Extend the hips" tells a client what to do at the top of a hip thrust; "squeeze" does not, and half of them squeeze the wrong thing.