To hail means to "call out". In HB Online, when you hail, it means you're opening up frequencies for communication between ships. For NPCs, they will say a predetermined phrase. For real players, you can chat.
Additionally, it's just a common term from universes such as Star Trek, where it basically denotes communication between ships and planets. So it makes sense why the word would be used in HB Online. If you're on a normal browser, take a look at this.
@Master Shouter That glitch no longer happens for me. Strange things can happen when you're short on memory, so when playing, just delete history and cookies periodically.