The 608 size ball bearings in particular are extremely low cost. Their 8 mm bore size almost forces us to mount them on:

Ball bearing sizechart pdf

The first digit of the model name refers to the kind of bearing, the second digit to its robustness. The third digit is usually the bore size in mm. Then follows the bearing type letters.

Bearing sizecalculator

The 626 and 624 bearings seem to cost more than 608 bearings, but M6 and M4 bolts and threaded bar cost less than M8 threaded bar and bolts, so it's almost a wash.

A ball bearing is a bearing used for low friction movements across an axis. They are cheap and widely available, mostly from the skateboard industry.