An impervious surface is one that can't be penetrated. The word is often followed by "to," as in "His steely personality made him impervious to jokes about his awful haircut."
Most of the sentences you'll run across using impervious will be followed by the word "to" and a noun. Things are often described as being impervious to physical assaults like heat, water, bullets, weather, and attack, but just as frequently to less tangible things, like reason, criticism, pain, and pressure. The word comes from Latin: in- + pervius, meaning "not letting things through." A common synonym is impermeable.