This command will increase the size of a selection in much the same way that the Shrink can be used to reduce the size of a selection area.
Here, you can set by how much your selection must be increased. The unit of measurement is pixel; you can change it by using the associated drop-down list.
When you grow a rectangular selection, you get a selection with rounded corners. The following image explains why:
To correct this, use the “Rounded Rectangle” command with a 0% radius.