The "thickness" (the height of a horizontal row, or width of a vertical row) is determined by the thickness of the thickest child slice on it.
So other children on the same row have some extra padding. This setting determines on which side to put that padding, or whether to center them

For example, in English text, if you use multiple fonts with different heights then different bits of text on the same line (row) are bottom-aligned to sit on the bottom of the line.