"Trigger finger" is the common name for stenosing tenosynovitis, a condition affecting the tendons and pulleys in the hand that are used to bend the fingers.
Inside the hand, tendons act like ropes that connect the muscles in the forearm with the bones of the fingers and thumb. The tendons in the finger run through a tunnel made up of a series of rings or pulleys, which holds the tendons close to the bone. To ease the tendon's movement, the tunnel has a slippery lining.
If the pulley where the finger joins the hand thickens and tightens around the tendon so it can't move freely, trigger finger or trigger thumb results. The tendon can sometimes grow a knot or nodule, or its lining can swell, and because it no longer glides easily through the pulley, it can produce pain or a popping sensation when the digit is moved.
Such a triggering sensation isn't like any other feeling. The tendon will worsen over time in a vicious cycle of swelling, inflammation, and triggering. Whenever it catches, it swells and inflames more extensively. If left untreated, the finger or thumb can become stiff and difficult to move or bend.