Open-access resources are those that can be accessed by anyone at any time without restraint. When the resource is abundant relative to the demand for it, an open-access regime may not only be unproblematic, it may actually be the best management regime since it involves so little oversight.
However when the resource is scarce, open-access resources may be subject to excessive use. Since the user's claim over the resource is only established by "the rule of capture", users have an incentive to harvest as much as they can as rapidly as they can. Instead of more conservative behavior preserving the resource in this circumstance, it could simply lead to resource degradation as other users simply increase their share.
Open-access resources should be distinguished from common-property resources. Common-property resources may allow many users to share the resource, but they may also do so in a way that restricts access (either formally or informally) to levels that are sustainable.