The bicep curl is the foundational isolation exercise for the front of the upper arm. You bend at the elbow against resistance, which makes the biceps brachii the prime mover while the brachialis and forearms assist. It's the movement most people picture on "arm day," and it's the generic curl a drafted program references by default.
What changes the exercise is the tool in your hands. You can do bicep curls with a barbell to load both arms heavily, but the starting weight is the thing — a barbell is less forgiving than dumbbell bicep curls, which let each arm move freely and fix side-to-side imbalances. Prefer constant tension? A cable bicep curl keeps the biceps loaded through the whole range, and a machine bicep curl locks your posture so beginners can focus purely on squeezing.
For whom: everyone from first-timers building base arm strength to advanced lifters chasing peak and detail. If a plan just says "bicep curl," treat it as a free choice — pick the variant that matches your equipment and how your elbows feel that day, and the rest of the program still works.