Humidity
Humidity is the amount of water held in the air around you.
Think of it this way
A jar holds marbles the way air holds water. Fill the jar halfway — that is low humidity. Pack it full to the brim — that is high humidity. A full jar has no room for more, just like very humid air.
Explaining humidity by grade level
The air around you holds tiny bits of water you cannot see. On some days the air holds more water, and your skin feels damp and sticky. A weather station can show when the air has more or less water in it. That sticky feeling means the air is full of moisture.