It is a layer that cannot be penetrated. Typically it refers to penetration by water, though it may sometimes refer to air. If water cannot permeate through a layer, that layer is impermeable.
Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid
Highly impermeable layers such as clay or shale are referred to as an aquitard. While permeable sand and limestone that can transmitt large amounts of water to a well are referred to as an aquifer.