[quote="Ad_Orientem, post:6, topic:342151"]
In general it means a Traditional Latin Mass with more ceremony than a Low Mass. A Low Mass (Missa Lecta) is said not sung, and can be done with only one altar server or even without one.
The unofficial term "High Mass" may refer, more specifically to:
-- Missa Cantata (Sung Mass) - what dshix describes above
-- Missa Solemnis (Solemn Mass, or Solemn High Mass) - same as the above, but with a deacon and subdeacon (or higher rank), and thus a little more ceremony
-- Missa Pontificalis (Pontifical Mass) - same as the above, but celebrated by a bishop, and thus yet more ceremony
-- Missa Papae (Papal Mass) - same as the above, but celebrated by the Pope, and thus yet more ceremony (hasn't been done in the traditional form in decades)
I don't know if the Pope could even celebrate a Missa Papae under the 1962 rubrics - don't they have specific roles for specific positions of the Papal court which no longer exist? I think the triregnum is part of the required vestments, as well. As Pope, he could modify these rubrics, but I doubt we'll ever see the Missa Papae again.