FrameLabel is an option for two-dimensional graphics functions that specifies labels to be placed on the edges of a frame around a plot.
FrameLabel -> None specifies that no labels should be given.
FrameLabel -> xmlabel, ymlabel specifies labels for the bottom and left-hand edges of the frame.
FrameLabel -> xmlabel, ymlabel, xplabel, yplabel specifies labels for each of the edges of the frame, ordered clockwise starting from the bottom edge.
Any expression can be specified as a label. It will be given in OutputForm. Arbitrary strings of text can be given as "text".
Labels for the vertical edges of the frame are by default written vertically. RotateLabel -> False specifies that they should be horizontal.
See The Mathematica Book: Section 1.9.3 and Section 2.9.5.
See also: AxesLabel, PlotLabel.