A truck is a nautical term for a wooden ball, disk, or bun-shaped cap at the top of a mast

Mast (sailing)

The mast of a sailing vessel is a tall, vertical, or near vertical, spar, or arrangement of spars, which supports the sails. Large ships have several masts, with the size and configuration depending on the style of ship...

, with holes in it through which flag halyard

Halyard

In sailing, a halyard or halliard is a line that is used to hoist a sail, a flag or a yard. The term halyard comes from the phrase, 'to haul yards'...

s are passed. Trucks are also used on wooden flagpoles, to prevent them from splitting.

Without a masthead truck, water could easily seep into the circular growth rings of a wooden mast. However, the grain in the truck is perpendicular to that of the mast, allowing the water to run off it.