The contact lenses are gas-permeable, so the eye can breath.
allowing fluids or gases to pass through
Note that a second sense of the word comes into play when the prefix "im" is added. Typically Impermeable means "not allowing fluids or gases to pass through", but it can also mean that someone or something is "not affected by negative things." For example, "She is impermeable to pain," or "He is impermeable to her tears."