As DirkS said, that's the SoC (System on Chip), or the main processing chip on the Pi computers (CPU and GPU, and some other things, but not wireless). Since manufacturing multiple chips costs more, RPT has been using the B0 chips on the 3B in recent production (haven't seen them on the 2Bv1.2 yet, but I wouldn't be surprised).
It's still clocked at 1.2GHz on the 3B (but likely can be overclocked, and it has the improved boot-loader). It's a non-issue for most people. The only potential problem would be for cases which act as heatsinks that make contact with the SoC, which is taller on the B0 stepping (Flirc Gen2 cases come with a thinner thermal pad for the 3B+/B0 SoC).
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?