Senbotsu means died/killed in battle, a bit like KIA. the idea behind the anime is that criminals, clinically insane and social misfits are no longer incarcerated for their crimes/conditions but are instead drafted into the millitary to serve as the fleets shock troops. Someone has to be the first into battle and as the first to engage the enemy usually has a slim chance of survival (think first to land on a beach, first to storm a castle, first to attack a bunker etc.) It might as well be societies trash that gets killed rather then brave outstanding citazens. Of course if these misfits survive the engagement, there is always next time... That's the background for the idea, the main characters are of course members of these shock units also known as Senbotsu Units as to coin the phrase 'they are already dead, they just don't know it yet!' Of course in true anime tradition a core of these misfits will become major heroes in the defence of their ship and of course will save the world... enjoy :)