What does letter B on B-1 fire extinguisher indicate?

B on fire extinguishers generally means that it can be used to put out flammable liquid fires. B1 means it will do so, but not especially well (1 is the lowest ranking).

A is for flammable solids, C is for electrical fires.

Usually the number appears before the letter, so 1-B rather than B1. Properly used, a 1-B extinguisher can be reasonably expected to put out a puddle of flaming liquid about 2.5 square feet in area. If you've never had specific training in using one, you probably WILL NOT use it properly; for flammable liquid fires, unless it's an extremely small fire it's almost always better to call 911 than to try to use a fire extinguisher on it (you're more likely to spread it than extinguish it).