What do the biceps do in arms?

Bicep translates from latin to mean "two heads" and this describes the most notable feature of the muscle. The biceps brachi is one of the muscle in the arm that acts on the elbow joint to suppinate...

it contracts

One on the right, and one on the left.

muscles work in pairs so when the bicep relaxes the tricep contracts, these actions create elbow extention (arm lowering) creating an objuce angle- lowering stage in a bicep curl

Move your arms towards themselves when pulling and to allow your tricep to pull your arms the other way when relaxing