Hello Doc
It's the minimum number of people/officers/members that must be present for business to be conducted in a club, political party, or other group.
For instance, if you were to set up a political party, you would probably draw up some rules; and one of those rules might state that 60% of members must be present, if any new policy is to be voted on. The 60% of members would then be the "quorum".
MrP