What does 'reef-safe' actually mean for a sunscreen?

There's no FDA definition. In practice, reef-safe labeling means the formula avoids oxybenzone and octinoxate, the two filters Hawaii banned. 100% mineral sunscreens using only non-nano zinc oxide and/or titanium dioxide are generally accepted as the safer choice for marine environments, though the science on nano vs. non-nano particles is still debated.