Airplane windows are actually made of three acrylic panes. The outer pane handles the full pressure difference between the cabin and the thin outside air; the middle pane is a backup in case the outer one fails; and that tiny "bleed hole" sits in the middle pane, letting a small amount of air pass through to equalize pressure between the panes. This means the middle pane only has to withstand pressure if the outer one ever actually fails, and it also helps prevent fogging by keeping the air between panes dry.