According to the somewhat controversial Swiss psychologist dude Carl Jung, archetypes are universal signs and symbols from the collective unconscious. They supposedly reflect common human experiences, like birth, death, motherhood, fatherhood, and whatnot.
Archetypes can be expressed in literature as settings, characters, images and situations. Literary critics like to talk about recurring patterns and motifs as archetypal. Think of them as a symbols that everyone can recognize (without a whole lot of effort involved).