For the M2 Rev A thru F:  These high grade bearings prevent damage to the X and Y axis belts and help keep them aligned.  Each M2 3D printer includes  two bearings sets ( one for each belt) . Each bearing set is mounted onto a separate idler bolt.  A purchase here gets you one idler bolt and accompanying bearing set.

For the M3/M2 Rev G thru H:  On the M3-SE we use two of these, and on the M3-ID we use three (one for each belt).  The M3 idler set has a geared pulley with a bearing inside that mounts on a bolt. A purchase here gets you one bolt and one geared pulley with a bearing inside.