The barbell curl is the heavy-loading option in the bicep curl family. Because both arms work together on one bar, you can move more weight than with dumbbell bicep curls, which makes it the go-to for building raw biceps strength and size. The primary mover is the biceps brachii, with the brachialis and brachioradialis assisting and your grip working hard to hold the bar.
The trade-off is that the bar fixes both hands in one path, so a strong arm can compensate for a weak one and the locked wrist position bothers some lifters — an EZ bar, or a swap to dumbbells, fixes that. Often simply called "curls" in the gym, it's a staple in strength and mass programs. For constant tension instead of a strength curve that eases at the bottom, use a cable bicep curl; to remove all body english, the preacher curl.
For whom: intermediate and advanced lifters chasing progressive overload on the biceps, and anyone whose program prioritises arm strength over isolation. Beginners can start here with a light bar, but dumbbell bicep curls are usually the gentler first step.