While at speed with a rate of hundreds of kilometer per hour and over 5,000 feet in the air, this pin-sized hole will ensure that the outer glass panel bears the most pressure so that if there comes a situation that causes added strain in the window, it’s the outside panel that gives out, still enabling us to breathe.
The plane windows are made up of three glass panels from inside. The outer and middle ones effectively resist the variance in pressure between the cabin and the atmospheric pressure outside, with the breathe hole making it sure that only the outer panel withstands the massive pressure.
More also, there is effectively a pair of glass panels outside (one is facing outside and another one in between this and the inner panel); thus it formed a pretty safe and unbreakable window.
Having read this, you can now go and watch the movie “Snakes on a plane”.