The Invincible attack is an attack that Mario or Luigi temporarily gain after they pick up the Star Power-Up item. and becomes Invincible Mario for a brief time until this Power-Up wears off.
As can be As presumed from the name of this attack type, nothing can harm a Mario brother. Mario and Luigi may kill other invincible foes such as Dry Bones and Thwomp. In this form the Mario brother is glittering like a rainbow and can run faster and jump higher. Oddly in this form Mario cannot damage Bosses without Being invincible/Fire or Invincible/Boomerang or so on.