We act socially, striving to maintain the identity we create for others to see. This identity, or public self-image, is what we project when we interact socially. To lose face is to publicly suffer a diminished self-image. Maintaining face is accomplished by taking a line while interacting socially. A line is what the person says and does during that interaction showing how the person understands the situation at hand and the person's evaluation of the interactants.
Social interaction is a process combining line and face, or face work.
Brown and Levinson use the concept of face to explain politeness. Negative face is the desire not to be imposed upon, intruded, or otherwise put upon. Positive politeness addresses positive face concerns, often by showing prosocial concern for the other's face. Negative politeness addresses negative face concerns, often by acknowledging the other's face is threatened. Of course, a person can choose not to threaten another's face at all, but when a face must be threaten, a speaker can decide how threatening he or she will be.