A quorum is the smallest number of board members necessary to legally conduct the business of your HOA. Most, but not all, groups define a quorum as a simple majority of the members. It should be defined in your HOA's bylaws.
Without a quorum, a vote cannot be taken, and the status quo can't be changed.
Unless a quorum is present, minutes should note be taken since official business of the HOA cannot be conducted.
Opponents to a proposal that would change the status quo can block the proposal by walking out of a meeting or not attending in the first place.
Usually, the rule that defines a quorum ignores any vacant seats. In this case, if a simple majority is required to have a quorum, and there are seven board seats, but there are only four members, all four would have to be present to conduct business.