Resistance training involves two types of movements, concentric and eccentric. Concentric movement is when the muscle shortens while producing force (contracting the muscle). This happens when you are raising the weight during a biceps curl.
Eccentric movement is when the muscle lengthens while producing force. For example, when you’re lowering the weight back down during a biceps curl. One can say that you’re often stronger in the eccentric phase of the lift since you’re holding back the weight.