It meant that volunteers basically come in two flavors. Most common the people that sign up for everything, show up sometimes, and promptly leave when the going gets tough. They like getting the credit for volunteering; the work, not so much. There are a lot of these people in fandom, and you can get some work out of them, but God help you if you actually give them some responsibility, because when push comes to shove (and it inevitably will) they'll fold like a cheap suit. These are the professional volunteers. They're good at volunteering for things, and that's about all they're good for. Make them badgers or give them grunt work, closely supervised by reliable staff, but don't actually give them a staff position. You want people on staff that you can rely on, and these people ain't it.
Less common and infinitely more valuable are the volunteer professionals. These people usually volunteer for a specific job, one that they're already good at and don't need to be trained into. Maybe some orientation as to who else they deal with on staff, but that's it. Usually these people have to be talked into volunteering and carefully looked after, because they work hard and attract attention from professional volunteers who are dying to unload some of their work on somebody who will actually do it. If you're a department head or a con chairman one of your most important jobs is to make sure this doesn't happen. Volunteer professionals are very good at what they do; they also tend to be cranky, difficult, opinionated (especially in their area of expertise) and, well, high-maintenance. But they're completely worth it. You can't run a volunteer organization without them.