A horizontal white bar between two horizontal blue bars makes up Juliet, which stands for "J" in the signal flag alphabet. You'll see that apparently the ICAO were fans of Shakespeare (LINK TO ROMEO). Ironically this Juliet is clad in blue, not Capulet red, although if you point that out to an old sailor they're gonna laugh at you and call you a nerd.
This is basically a "STAY AWAY FROM ME" flag. If your ship is on fire or capsizing and you have dangerous cargo, or if you're leaking that dangerous cargo into the water, this is the flag for you. I'm really not sure what else to do in that situation though, you're on your own.
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