Confused is a debuff which causes sideways controls to reverse: ◀ Left will move right, and ▶ Right will move left. Additionally, ▲ Up will also act as ▼ Down, and ▼ Down will do nothing. When applied to enemies, a question mark appears above them, and causes them to move away from the player rather than towards them. Enemies with ranged attacks will be unable to use that attack. The debuff can be prevented with the Trifold Map (and by extension, anything crafted from it), which is dropped by most enemies that cause the debuff.
About 70% of enemies and the entirety of bosses in Terraria are immune to the Confused debuff. Many of the vulnerable enemies include slimes, bats, and most fighters.