It’s easy to miss it, except when a hollowed-out snowflake of frost forms near it. This tiny hole is called a breather hole or a bleed hole, and it serves an important safety function.
Typically, a cabin window has three panes. The innermost pane serves the purpose of protecting the next one. The middle pane (with the breather hole in it) and the outer pane are more important.
The breather hole regulates the amount of pressure that passes between the window’s inner and outer panes. This system ensures that the outer pane bears the most pressure so that if there were a situation that caused added strain on the window, it’s the outside panel that gives out (meaning you can still breathe).
The breather hole also keeps the window fog-free by wicking moisture that gets stuck between the panes.
Here are some graphics that explain further.