Lima is made of 2 yellow squares and 2 black squares in a checked pattern, and it stands for "L" in the signal flag alphabet. It looks very Pittsburgh, which is a city, and Lima is also a city, so I think I win the prize for biggest stretch mnemonically. You'll see this flag a couple times on the Lifelines hat too.
This is not a great flag to see. If a ship is flying this flag at port, that means they're in quarantine. Now, hopefully that's just some sort of rule, that they have to wait for a bit before entering a new port. But, it COULD mean that the ship is full of zombies. So, y'know, approach at your own risk.
If the ship flying it is at sea however, this means that you should stop your vessel immediately. It doesn't say if that means there's danger on the seafloor, or if it means "Stop or we'll shoot", but just stop either way.
Used with numbers, this flag signifies lattitude, with the first numbers being degrees and the last being minutes. Based on the above uses for it though, you should probably string the whole thing together with the numbers before you put it up. Just to keep everyone else at ease.
Check out the rest of our Signal Flag explanations!