A floatstick is a device to measure fuel levels in modern large aircraft. It is made up of two tubes, one enclosed within the other, around which is a ring-shaped float that contains a magnet. There is also a magnet attached to the top of the floatstick. The floatstick is withdrawn from the bottom of the wing until the magnets stick, indicating the level of the fuel.
Older aircraft use a dripstick.