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To understand density, we first have to understand hot and cold. Imagine all the air molecules are at a party. If they all stand around in one place, the party is cold. But if there's good music, they dance, they have energy, the and room gets warmer. Air molecules that move more have more heat.

But these molecules are bad dancers. they keep running into each other! So they step further and further away from each other, which makes the room less packed, less dense. 

Hot air is less dense than cold air, because the molecules are in motion, bouncing off each other, causing the air to expand, and to be less dense.