What part of the ear pops?

Answer 1

Ears pop because the atmospheric pressure becomes greater or less than the pressure inside the ear. While the ear drum forms an air-tight seal between the middle ear and the outside atmosphere, the eustachian tubes allow air pressure to equalize between the environment and the middle ear.
Your ears pop in air planes because the air high above the surface of Earth is less dense than air near the surface.

As you ascend in an airplane and the air pressure decreases, the air trapped in your inner ear will push your eardrums outward.

Your body equalizes the pressure between your inner ear and the atmosphere by allowing some air from your inner ear to escape. The air escapes through the Eustachian tubes, two small channels that connect the inner ears to the throat on each side.

When these channels open, you feel the pressure release and you hear the change because it's in your ear. This is the "pop."

On the way down, the air pressure increases, and the extra pressure pushes the eardrums inward. The pressure will, again, have to reach an equilibrium.
The air pressure becomes less as you go higher and higher due to gravity, air etc, therefore our bodies automatically adjust to compensate for this. The "popping" is sort of like a valve which adjusts the air pressure in our ears.

Answer 2

When your ears pop it is your ears way of adapting to the air pressure change. The air inside your head is lower then outside of it.

Answer 3

It means that gas is being released from your joints and rebuilds in 20 minutes.

Answer 4

The ear "drum" a thin membrane inside the ear "pops" when pressure is too great to release said pressure