Reef safe means whether it can go with corals and inverts. Dwarf lions are "reef safe" but they are not "invert safe". They will not touch a coral, however, a shrimp is a quick dinner. Some people consider them not reef safe because you cannot keep them with typical reef inhabitants, such as shrimp.
A puffer or trigger is "not reef safe" because it will tear through corals, inverts, etc.
If there is a fish in question, just ask.