Something can be called penetrable if substances can pass through it. A coffee filter is penetrable, for example, because it allows water to drip through it.
Your curtains are penetrable if they let sunlight peek through, and a country's borders are considered penetrable if they allow people to pass through them unnoticed by guards or customs officials. In basketball, having a penetrable defense is bad news — it means it allows the opposition's players to get right up to the basket, where they can easily shoot. Sometimes penetrable means "understandable," as in a penetrable poem or penetrable song lyrics.