Dew is moisture caused by condensation of water vapor in the air. Dew is what gets your feet wet when you walk across the grass on a summer morning.
Think about a summer night — the sun goes down and the air begins to cool, along with everything that is outside, like grass. Then the sun comes up. The air heats up again, more quickly that the objects. This causes condensation — dew — on grass, plants, car windshields, and so on. Dew quickly evaporates in the sun. That's why you really only see and feel it early in the morning during summer.