The //ML and //MR commands are used to affect margins:
/ML12//MR12/This paragraph began with a //ML12// and a //MR12// command. Notice how the left and right margins have been moved in by 12 quarter widths (a quarter width is equivalent to a quarter of the width of a typical character in the current font).
/ML-12//MR-12/This paragraph began with a //ML-12// and a //MR-12// command. This returns the margin settings to normal as you can see by the formatting of this paragraph!