A trigger finger or thumb is when your finger or thumb feels like it catches or locks and then is difficult to straighten.
After a period of time and some pain, it then releases, gives way and straightens. It is due to a thickening or swelling of your tendon that gets caught as the tendon passes under its pulley or sheath. It can be from arthritis or overuse of the tendon. It usually occurs at the A1 pulley of the flexor tendons at your knuckle joint or at the MCPJ of your thumb.