It's RAF terms I think.
Mass brief is a briefing to large (mass) group with regards to the air exercise that will be flown, including the aerodynamic considerations. Can be up to 1 hour.
Board brief is a 1 to 1 briefing by instructor to student just before the flight. It only covers the air exercise, which the student should have already learned during the mass brief. Normally max 15 minutes.
However, in "civilian" world, we still use the term mass brief, but not necessarily to a large group of students, but to mean that the briefing is about the air exercise including aerodynamic considerations.
Board brief is only air exercise.