PageBreakBelow is an option for Cell which specifies whether a page break should be made immediately below the cell if the notebook that contains the cell is printed.
A setting of Automatic specifies that a page break should be made if necessary.
A setting of True specifies that a page break should always be made, while a setting of False specifies that it should never be made.
See The Mathematica Book: Section 2.10.9.
See also: PageBreakAbove, ShowPageBreaks.