Disguise lets you cast a creature face-down for as a 2/2 with no name, types, or abilities. Later, you can turn it face-up by paying its disguise cost, revealing the real creature. Crucially, disguised creatures have ward , so opponents must pay to target them—making them harder to kill than face-down morphs.
The common mistake: turning a creature face-up isn't casting it and doesn't use the stack, so it can't be countered and opponents get no window to respond before it's already face-up. However, you can only flip during a moment you have priority. Also, the face-down creature is colorless with mana value 0, which matters for effects that care about those values.
In Commander, disguise adds bluffing and mana flexibility—threatening combat tricks or flash-style reveals—while ward protects your investment from cheap removal and sacrifice effects.