ICBM is one of four satellites introduced in CI4. It's three Cold War ballistic missiles
It's full name would be "Instant Chicken Barbecue Missile." (Just to fit with the chicken puns)
Actually, it's real name is "Intercontinental Ballistic Missile"
Although quite ineffectual against single targets, its exceptionally large blast radius can damage dozens of enemies at one time. ICBM rockets unfortunately travel very slowly, so their strategic use is of prime importance.
With it, the tip is that best probablity to use ICBM is large waves, with many enemies, because with even only one missile from it, dozens of enemies will be killed fast.