Sail can mean a few different things:

1. A large piece of fabric attached to a boat's mast that catches the wind to move the boat forward. Sails work like wings: as wind flows around them, it creates force that pushes the vessel through water. Ancient sails were square and made from woven reeds or animal skins, but modern sails are often triangular and made from synthetic materials that are strong and light.
2. To travel by boat, whether powered by wind, engine, or oars. When your family sails to an island for vacation, you're traveling by boat. A cruise ship sails from port to port. The word works even if the boat has no actual sails.
3. To move smoothly and gracefully, as if gliding. A paper airplane might sail across the classroom, or an eagle might sail on air currents without flapping its wings. When you finish your homework quickly because you understood everything, you might say the work sailed by.

The noun form appears in phrases like set sail, which means to begin a voyage, whether on water or metaphorically. When explorers set sail for unknown lands, they're starting an adventure into the uncertain.