The drawer guide brackets hold in place the monorail that your plastic drawer box rides on inside your kitchen cabinet.

Sold in pairs (1 front, 1 back).
 

Unscrew the existing front and rear guide and remove the monorail.  Attach the new front guide onto the cabinet and insert the front of the monorail into the front guide.  Slip the new rear guide onto the other end of the monorail and reattach the new rear guide back inside the cabinet.