GNU Emacs Lisp Reference Manual

2.4.11: Overlay Type

An overlay specifies temporary alteration of the display appearance of a part of a buffer. It contains markers delimiting a range of the buffer, plus a property list (a list whose elements are alternating property names and values). Overlays are used to present parts of the buffer temporarily in a different display style. They have no read syntax, and print in hash notation, giving the buffer name and range of positions.

See Overlays, for how to create and use overlays.