Air pressure drops as you get high. But airplanes are designed to maintain air pressure at a safe level inside the cabin. So the air pressure outside an airplane is much lower then it is inside.
The difference in air pressure puts a lot of physical stress on airplane windows,
Air plane windows consists of Three separate panes
Outer Middle Inner
The outer panes deals with this air pressure difference. Thanks to the tiny hole in the middle pane.
known as the "BLEED HOLE". Its primary purpose is to balance air pressure, There's a small gap in the middle and outer panes. The BLEED HOLE allow pressure to balance between the passenger cabin and the air gap.
Meaning the outer pane takes While the middle pane act as
on the pressure a safe-fail
The inner pane is non-structural but it help protects the middle and outer panes from damage caused by passengers. The BLEED HOLE also releases moisture from the air gap this prevents the window from fogging up or frosting over .
