Morph lets you cast a card face down as a 2/2 creature for {3} instead of paying its normal cost. While it sits face down, your opponent sees only a nameless 2/2 and cannot tell what it really is. Later, any time you have priority, you can turn it face up by paying its morph cost, and it becomes its true self — real name, types and abilities.
That hidden information is what makes morph matter. You can attack with a 2/2 that might be something harmless or might flip into something far larger, so blocks become a guess. Turning face up is also a surprise: a creature that suddenly gains a removal ability, a bigger body or a triggered effect can swing a combat or answer a threat at the moment it counts.
