For web servers, this could mean authentication of the server, as with server identification certificates. A resource is accessible and usable as intended and when needed. In the real world, various problems beset the qualities that we desire for our resources. Attacks are intentional acts to subvert privacy, undermine integrity, fake or deny authenticity, steal resources, or simply to deny use. This reflects the dark side of human nature.
Errors are unintentional acts by users, staff, maintenance personnel, etc.
Yes indeed, to err is human. Malfunctions are glitches, bugs, breakdowns, natural disasters, and the like. They are Nature's way of letting us know we can't control everything. The three main areas of security activity are prevention, detection, and recovery. Activity must be balanced among all three areas to be effective.