Suppose you have a large collection of N numbered copper coins. However, one of them is nickel but looks exactly the same as the rest. How do you find out which number the nickel coin has?
Classical computer: Try each of them in turn to see if it is magnetic, this takes on average N/2 tries.
Quantum computer: Put them all in a bag together with a magnet on a string, shake a bit and pull out the magnet plus the nickel coin. If it doesn't work the first time try again. This takes only one or a few tries. The magnet interacts with all the coins at once.