The difference between a qubit and a classical bit is the states they are allowed to be in. A computer works by 1's and 0's, literally this is the on and off state of a bit. A qubit can be in three different states: on, off, and an undetermined state of both. This is a game changer.
Although quantum computers won't be the ultimate gaming machines, they will change the world of security and mathematics. They are so mathematically powerful, they have the ability to "brute-force" large encryption keys using special algorithms many times faster than classical computing.
Everything that runs through secure connections online, like buying things with your credit card, rely on the premise that other people will not be able to crack your encrypted tunnel. Almost every single form of security online would need to change once quantum computers are built and advanced. Quantum computers would be able to crack through 128-bit keys as quickly as your classical computer can use it. It's pretty amazing.
From what I understand, quantum computers are also able to use mathematical algorithms that were not before possible by classical computers. That means they'd be able to use formulas to break certain encryptions, where brute-force attacking was the only option before.
A lot of this is speculative for me. I don't really know what the implication of a qubit in quantum computing means, because in the nature of the large scale, we have no idea what it means to be in the "on" and "off" state at the same time. All I know is that big changes are coming because of the study of quantum physics.
Pic related. It's a preliminary quantum computer.