The biceps brachii is one of the most often trained muscles. stereotypically most guys want big biceps - and in practice this is often true. As is commonly known, the main function of the biceps brachii is elbow flexion (what occurs during a curl exercise). However, the bis have far more responsibilities than just curls.
The bis are comprised of a long and short head, hence the name bicep. Both heads originate from the scapula and insert into the radius.
The short head of the biceps is responsible for curling motions. Additionally, the short head is also responsible for forearm supination, which is the twisting of your forearm until your palm is facing the ceiling. The short head also assists in shoulder flexion (raising your arm directly out in front of you), shoulder horizontal adduction (when your arm is directly out to your side and you bring that arm forward), and shoulder adduction (when your arm is directly out to your side and you draw that arm in toward your torso).
The long head of the biceps shares some of its functions with the short head, including elbow flexion, forearm supination, and shoulder flexion. The long head also aids in shoulder abduction, which occurs when you raise your arm out to the side. The long head only assists in shoulder abduction when the shoulder joint is externally rotated.
Because of the short and long head’s actions on the shoulder joint, curling movements where the shoulders are flexed (arms in front or above you) preferentially recruit the long head.