This slider slides up and down the tower, controlled with the timing belt. The up and down movement gets translated through the arms to a horizontal movement of the effector (hot end). This slider is designed for use with the MakerSlide and has 3mm holes to mount the MakerSlide bearings. Either 3 or 4 bearings can be used, based on your requirements for precision or cost.
This part is simply the aluminum slider. The other pictures show the rollers and other hardware installed on the slider, but they are not included with this particular part. The rollers can be purchased from http://www.inventables.com