TableAlignments is an option for TableForm and MatrixForm which specifies how entries in each dimension should be aligned.
TableAlignments -> , , ... specifies alignments for successive dimensions.
For dimensions that are given as columns, possible alignments are Left, Center and Right. For dimensions that are given as rows, possible alignments are Bottom, Center and Top.
The default setting TableAlignments -> Automatic uses Left for column alignment, and Bottom for row alignment.
See The Mathematica Book: Section 2.8.8.
See also: TableDirections, RowAlignments, ColumnAlignments.