In classic computers, their is a small unit called “bit”. It can be either “on” or “off”.

In quantum computers, the small unit is called “Qbit” (Q for quantum). It can be “on”, “off” or “I don’t know “ state.

In layman terms, Quantum mechanics has a state called “I don’t know” which basically means you don’t know until you know. It’s like when you were in school, your parents wants to know if you are studying or not. They have to peep into your room to really see if you are studying or not. You may be studying, you may be dancing, you may be watching p@rn etc. At minute scale lot of things behave like you in the room. We don’t know until we peep to see what they are doing.

Coming back to quantum computers, having that third state will help in calculations taking lesser time than classic computers.

Hope it helps.