These linear ball bearing is sort of the opposite of the radial ball bearings you may be familiar with. Its intended to slide along an 8mm linear shaft, rather than rotate around it. These are very slim, and good for when you want to attach a motion carriage onto a railing without adding a lot of weight.
These are very basic bearings, they’re meant for a stepper-motion controlled setup so they’re not ultra-smooth.
Long Type Linear Ball Bearing Bushing