B10 is the engineering term for predicted engine life from the manufacturer. It's usually used in reference to diesel engines but I believe it applies to gas as well. The 10 stands for the mileage or gallons fuel burned at which 10% of the engines produced will have catastrophic failure.
B50 is the same concept except the it is the point at which 50% of the engines produced fail.
These numbers are usually the result of extended testing.
Example: The B10 life for a 6.0 liter Powerstroke in a Ford F-250 is 300k miles. Thus, 10% of the enigines will have failed or required a rebuild by 300k miles.