You probably know by now that typical computers function by using 1s and 0s, using binary maths. The transistors in them are either off (0) or on (1), with data being held as binary digits (bits).
In quantum computing, quantum mechanics form the basis of the machine. Rather than bits and bytes, quantum computers use quantum digits (qubits). I have to confess that I don’t understand the maths involved, but the two things to bear in mind are these:
There are more than just 1s and 0s: qubits can be in multiple states at the same time
Viewing the state of a qubit changes it
What these mean is that quantum computers have the potential to be incredibly fast, but it’s difficult to make use of their multiple states because looking at their state changes them.
Some organisations eg IBM have built small prototype quantum computers, but the technology is in its infancy. It will probably be several years before this sort of processing becomes commercially available.
When they are finally built, processing speeds will be massively increased, which also means that existing cryptography techniques will be at risk because even brute force attacks will be able to be carried out so much faster. A new form of quantum cryptography will have to be developed and implemented.