Quantum computers use quantum physics, which allows them to perform a huge amount of calculations simultaneously, making them multiple times more powerful than classical computers. This means they could break encryption schemes we currently use in just a few hours (or minutes), whereas it would take a traditional computer thousands and millions of years to do so.
While large-scale quantum computers are currently only used for research purposes in universities, supercomputer centers, and scientific labs, when they do become widely available and powerful enough, they would be able to break all public key schemes currently in use. This day, i.e., when quantum computers would "break the internet," is referred to as Q-Day.
