Tan Tui (Springing Legs or Snapping Legs) is a basic kung fu form (a choreographed shadowboxing sequence). It is a very good conditioning exercise, good for cardiovascular health, balance, rhythm, coordinated movement, stretching and strengthening the muscles, and developing long punches and kicks. It can be done purely for health or as supplemental training to a martial arts regimen. Of course, to learn practical self-defense, you cannot just practice drills and shadowboxing; you need sparring (practice fighting). As part of the Northern Chinese Long Fist system, it emphasizes long movements, punches, and kicks.
Tan Tui was originally created by Muslim Chinese and practiced by those who served as guards protecting the trade caravans going in and out of China following the Silk Road leading through Central Asia. It is now a core part of most Northern Shaolin Kung Fu curricula. There are various styles but the one below is very similar to the one my school practices, with the main exception being that their kicks are lower.