These are the material strength property classes. They are designated by a number. The metric grades are 4.6, 4.8, 5.8, 8.8, 9.8, 10.9 and 12.9. The higher number is essentially a harder bolt. They also take a different maximum torque (which is also based on length of the bolt). They vary in their proof load stress (for carbon steel from 225 to 970 MPa), minimum tensile yield strength (from 240 to 1100 MPa), and minimum tensile ultimate strength (from 400 to 1220 MPa).
You ask, you learn.... you don't, you're dumb.
wow... thanks for the info! so where would one acquire these bolts? would a hardware store carry them? and do they have the strength printed on the bolts so you know what to get?