It's basically saying "if you try to enact gun control, we will shoot you", make of that what you will.

Yes, The muricans have a couple of rallying cries around the right to own guns and "standing your ground." The conservatives larp as tough guys.

It comes from "molon labe" (which you'll also sometimes see) which was what the Spartans said when told to lay down their arms or face certain defeat by the Persians at thermopylae. Spoiler: they came and took them.