The B3 stepping you are probably referring to is the B3 Stepping of motherboards which used the Intel P67 or H67 Chipsets.
Like constepatdyak mentioned, versions before the B3 stepping (eg. B2 stepping) had a hardware problem with the SATA-II ports, which had the potential to fail over time (more than the possibility was normally).
The B3 stepping fixed this problem.
This happened more than a year ago, pretty much every P67/H67 motherboard on the market now should be B3 or higher, but it doesn't hurt to check before you buy.
In general, a stepping is basically a revision or version number (constepatdyak pretty much covered it).