That is the fix for a stem that develops a squeek from the stem contact the inside of the steerer tube. Often this is most noticble when we get out of the saddle and really put some twisting and side to side stress on the handle bars.
Pull the stem from the steerer tube.
Using any grease (automotive or other) put a coating over the portion of the stem that is inserted into the steerer tube.
If there's any rust on the outside of the stem, it's a good idead to clean it up with some emory cloth first.