Gravity affects the accuracy of watches. The effect is small, but it adds up. Since it's impractical to wear your watch only in space, a tourbillon -- pronounced "ter-billion" -- is used to counter gravity, keeping the watch ticking away accurately as the seconds accrue.
The tourbillon is a sort of cage that holds the movement and escapement in place regardless of how the watch is turned. The tourbillon itself turns to nullify the effects of gravity on the intricate parts inside.
The tourbillon is considered one of the most difficult and complex parts of the watch to make. IT is frequently mounted on the face of high-end watches, the better to display the fine craftsmanship.