The core exercise described is a sit-up: you lie on your back with your knees bent and raise your torso toward your knees. It is a traditional bodyweight movement that trains the abdominal muscles along with the hip flexors.
A standard sit-up begins with the feet planted, knees bent, and hands placed across the chest or behind the head. The upper body then curls upward until the torso moves substantially toward the thighs before returning under control. Breathing steadily and avoiding momentum makes the movement more demanding and easier to perform consistently.
The most common mix-up is the crunch. A crunch uses a shorter range of motion: the shoulder blades lift from the floor, but the lower back generally remains down. A sit-up raises much more of the torso. Leg raises and planks also train the core, but they involve different movement patterns rather than lifting the upper body toward the knees.
Sit-ups are often included in fitness tests and general conditioning routines, although core strength can also be developed with safer or more varied exercises such as dead bugs, planks, and controlled curl-ups.