First of all: there is no standards definition what B10 really is, especially what "B" stands for. Most - probably by copying each other - believe that it means "Bearing".
B10 is the expected time or the number of operations after which 10 % of the supervised/tested products will have failed. This value is defined by IEC/ISO 13849-1 for functional safety. Usually B10 is used for electromechanical parts.
An other - probably equivalent - value for "normal" products might be L1 (Does Hirschmann offer L10 values for their products' reliability?).
Hirschmann publishes MTBF values for their products, which is a statistical value calculated by an algorithm based on the implemented parts' values.