What is time dilation a simple explanation ?

Time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field.

 When a gas is cooled, it contracts. In a sealed environment, the pressure would therefore reduce, as the molecules try to get closer together, that is, push outwards less. But in the atmosphere, the closer-packed molecules (or atoms, of course) make the air more dense, therefore heavier.

If you were at a higher elevation, the cooling is caused by the reduced pressure causing the molecules to move further apart. The temperature reduces about 1 degree Centigrade for every 100metres gain in elevation.

Any good?