There are two aspects to this quote*:
First, Open Sea PvP is a very low restriction PvP system. Characters can be attacked by virtually anyone, and they can most certainly be ganked. That's the nature of the system, and we're not changing that system. So we don't want to hear any crying about it. War's not fair. Open Sea PvP is war. Open Sea PvP is not fair. I recommend trying to figure out how to make it not fair in your favor.
Second, it's a reference to the fact that it's only a game, and not something worth crying about. I know MMORPGs are very immersive games, and many people poor a lot of time and energy into these games. I'm really happy that players are that excited about Pirates, but at the end of the day, it's important to remember that it's still a game. It's not worth losing your temper, or getting upset over. It's just a game.
In short: there's no crying in the red circle.
*It should also be noted that the line is a reference to a quote from the movie "A League of Their Own." While it doesn't really impact the meaning of the quote, it does make it funnier to imagine a scruffy Tom Hanks shouting "There's no crying in the red circle!" At least, it's funny to me.