The margins are the normalized divisions of the texture; e.g. where does it slice along the left, top, right and bottom sides of the image, creating 9 slices.
A margin of 0.25, 0.25, 0.25, 0.25; on a 100x100 brush would result in corners that were 25 pixels large, the sides would be 25x____ or _____x25, depending on vertical or horizontal, and the remaining center area 50x50 pixels would stretch across the widget’s remaining surface.