A mast is a pole that rises vertically from a ship and supports the sails. Really big sailboats have more than one mast. A mast is also another name for flagpole.
The mast has an important job — to support the sails, which allows the wind to propel the ship. Other types of masts are used to support flags and called flagpoles. If you've ever seen a flag at half-mast, it's been lowered down the flagpole to show respect for someone who died. Mast is an old word meaning “pole or rod.”