This is an injury to the ulnar collateral ligament of the metacarpophalangeal joint of the thumb. A fall while skiing with a stock getting caught on the thumb, and pulling it away to the side is one mechanism of injury, and the reason for the name. In history, it was also called Gamekeepers Thumb, as game keepers used to wear out the ligament when breaking the neck of pheasants or hares.
The critical problem in Skiers thumb is that when the ligament tears, it folds back on itself, and is then held in that folded position buy overlying soft tissue. In that position the ligament can never heal. The joint is forever unstable, painful, and develops arthritis.
Surgery is best for acute tears. The ligament can be robustly repaired, and usually early unloaded movement can start at a week with a splint for protection.
Less commonly, the radial collateral ligament is torn badly enough, that it needs repair.