The drive socket (also referred to as the mushroom or sprocket) is the part on top of the blender base. When the teeth of the drive socket are worn, the part can easily be replaced.
When the teeth are worn, it is important to check the teeth of the bearing of the jug that is used with the blender. When the drive socket's teeth are worn, it is likely that the teeth of the bearing are worn too. Find the blade assembly here.
Check the serial number of your blender (at the bottom of the base) to order the part compatible with the serial no 4 blenders, or the series starting with serial no 5.