When this command is pasted into a command block (and powered by redstone), it will kill all entities!
This includes players, mobs, dropped items, armor stands, ender crystals, exp orbs, lead knots (tied to fence post), paintings, item frames, shot arrows, thrown snowballs, thrown ender pearls, thrown eyes or ender, ghast and blaze fireballs, thrown splash potions, thrown exp bottles, wither skulls, primed tnt, falling blocks, firework rockets, shulker bullets, dragon fireballs, splash potion and lingering potion effect clouds, boats, and minecarts.
Important tip
If there is a specific player you don't want to be killed, this can be easily configured.