Trigger finger can present with stiffness, catching, and potentially locking of the finger in a flexed posture.
Normal flexion and extension of the fingers requires efficient movement of the flexor tendon with the flexor tendon sheath. This is strong tube-like structure holds the flexor tendons in close approximation to the bones within the fingers. The lining of the flexor tendon sheath is smooth: any disruption of this surface results in a loss of the normal efficient gliding of the flexor tendon.
Trigger finger is probably caused by inflammation and swelling of the soft tissues that coat the flexor tendons, combined with localized wear and tear in the flexor tendon sheath. In trigger finger the flexor tendon catches on the edge of the flexor tendon sheath, this can cause pain, stiffness, and locking.