means The implied or underlying meaning beneath what is literally said or written — the message you're meant to read between the lines.

from A modern compound of the Latin prefix 'sub-' (under, beneath) and 'text' (from Latin 'textus,' literally 'a thing woven,' from 'texere,' to weave — the same root that gives us 'textile'). So a text is woven words, and subtext is what lies woven beneath them. The word entered English in the early 20th century, popularized especially in theatrical and literary criticism — notably through the work of Stanislavski and his followers, who used it for the unspoken thoughts and motives running under an actor's lines.