A safe harbor is a provision of a statute or a regulation that specifies that certain conduct will be deemed not to violate a given rule. It is usually found in connection with a more-vague, overall standard. By contrast, "unsafe harbors" describe conduct that will be deemed to violate the rule.
For example, in the context of a statute that requires drivers to "not drive recklessly", a clause specifying that "driving under 25 miles per hour will be conclusively deemed not to constitute reckless driving" is a "safe harbor". Likewise, a clause saying that "driving over 90 miles per hour will be conclusively deemed to constitute reckless driving" would be an "unsafe harbor". In this example, driving between 25 miles per hour and 90 miles per hour would fall outside of either a safe harbor or an unsafe harbor, and would thus be left to be judged according to the vague "reckless" standard.