Water droplets that form on grass and leaves in the morning are called dew. Dew develops when exposed surfaces cool enough for water vapor in nearby air to condense into liquid droplets.
On clear, calm nights, grass, leaves, car roofs, and other exposed objects can lose heat by radiating it toward the sky. If a surface cools to its dew point—the temperature at which air becomes saturated—water vapor begins changing into liquid water. Thin leaves and blades of grass cool especially efficiently, which is why dew is often easy to see there at dawn.
Dew is different from rain and fog. Rain falls from clouds, while fog is a cloud made of tiny droplets suspended near the ground. Dew forms directly on a surface. Frost is the below-freezing counterpart: when the surface is cold enough, water vapor can form ice rather than liquid droplets.
Dew is most likely when humidity is high, winds are light, and skies are clear. Wind can mix warmer air downward, while clouds can reduce radiational cooling, so either condition may limit dew formation.