Lots of exercises require us to keep a neutral spine. In order to achieve this, “Bracing” is a commonly used technique.
The usual muscles involved include the abdominals in the front and the spinal erectors in the back. Pelvic floor muscles from below also come to the party, and if we “breath in“, the diaphragm will add to the overall stability.
This natural girdle is what we look for before moving heavy boxes around, or practicing deadlifts. Core exercises, like abdominal curls or planks, usually require bracing to maintain good form, and avoid potential future back pain.