To simplify the idea dramatically, a quantum computer effortlessly outperforms a conventional computer by operating in a totally different way. Your personal computer interacts with data by representing it as "bits" – ones and zeroes. But a quantum computer deals in "qubits," which represent data as ones, zeroes, or the quantum state between the two. With the ability address more data at a time, quantum computers become tremendously powerful tools for a number of fields.
This might sound troubling to the Bitcoin world, as it's built upon cryptography to function properly. Bitcoins are generated only as computers break codes to unlock them, and the time required to break these codes is figured in to controlling inflation and stability. But it seems that if these super-powered quantum computers started crunching numbers to earn Bitcoins, they could throw the currency out of balance. Furthermore, there's the idea that they could even break Bitcoin security entirely.