Megamorph lets you cast a creature face-down as a 2/2 for (no abilities, no name, no color). Later, you can turn it face-up by paying its megamorph cost, revealing the real creature. Unlike regular morph, when a megamorph creature flips up it gets a +1/+1 counter, making it slightly bigger than its printed stats.
The common mistake: turning a creature face-up is a special action, not an ability—it doesn't use the stack and can't be responded to. Opponents can't kill it in response to the flip itself. Also, the face-down 2/2 is a legitimate creature; if it dies face-down, it just goes to the graveyard face-up with no triggers.
In Commander, megamorph is a niche but flexible tool—it dodges sorcery-speed removal, hides information, and lets you deploy threats early then reveal them at instant speed for combat tricks or surprise blockers.