How can a window screen let air in but keep many insects out?

A window screen is a mesh of thin strands with many small openings between them. It covers a large space without making that whole space solid.

Air can move through the open gaps. An insect wider than a gap cannot pass straight through it. The mesh therefore lets a breeze enter while blocking many larger insects.

The size of the openings matters. A very small insect may fit through a mesh that stops a bigger one. The screen works by its physical gaps, rather than by making the air disappear or frightening every insect away.