ColumnsEqual is an option for GridBox which specifies whether all columns in the grid should be assigned equal width.
The default setting ColumnsEqual->False determines the width of each column from the widest entry in that column.
ColumnsEqual->True makes all columns the same width, with the width determined by the widest entry in the whole GridBox.
See The Mathematica Book: Section 2.8.11.
See also: ColumnWidths, ColumnAlignments, ColumnSpacings, RowsEqual, MatrixForm.