Apparent wind indicator
An apparent wind indicator in yacht racing, mostly called a windex, is a small weather vane or arrow that gives an indication of the current wind direction.
When the object where the windex is connected to (e.g. top of a mast) is moving the windex moves more towards the direction of the movement. Therefore, it does not shows the true wind direction but the apparent wind direction.
Sailors use this device to get a clue if they can point higher or to set the sails.