Remember this word by what it rhymes with: helmet. A helmet is to one’s bed head as a pelmet is to one’s curtain hardware: it hides the unattractive bits. Yes, a pelmet is a decorative piece that is placed above a window to 1) conceal curtain fixtures, and/or 2) add to the décor. It can be simple or drastically ornamental, rigid or soft to the touch. It is similar in appearance to a cornice or valence.
See also: cornice, valance