Everything. 

A homeowners association is, by definition, a membership organization so that, if the board walks out, there are members left with an interest in the organization and the authority to pick up the pieces.  The resigning directors ought to find individuals who are willing to take over the responsibility and give them all of the organizational records and other documents that they need to do so.  The new directors can determine what documents are actually necessary to do the job, but should be given all of the historic records that should be retained permanently.

Directors have a fiduciary duty to the organization and it would be a breach of that fiduciary duty if, for whatever reason, they all decided just to pick up their marbles and go home, without finding or starting a process to find individuals to continue the organization.   It would be hard for those resigning to argue that they had acted in good faith if they failed to assure that someone was in place to take over the governance responsibility and failed to turn over the organizational records and other documents the new people needed to do so. 

In life, the last ones out of the building generally have the responsibility to turn out the lights and lock the door. In this case, the resigning directors have the duty to assure that they are not the last ones out.