Quantum physics is a branch of physics that helps us to understand how the things that make up atoms work (protons, neutrons, electrons etc.). It uses lots of complicated mathematical equations to explain things that can’t be explained by classical physics.
One area of quantum physics is the Copenhagen Interpretation. This uses mathematics to show how likely it is that a subatomic particle is occupying a given location, without observing it. If there is a 50% chance that a particle is in one place at a given time and a 50% chance it could be in a different place at the given time then we can think of the particle as being in both places simultaneously, and this could be used in mathematical calculations. Once we observe the particle we know where it is and it ceases to have a dual nature.
Further reading: How to Teach Quantum Physics To Your Dog by Chad Orzel (https://www.amazon.co.uk/How-Teach-Quantum-Physics-Your/dp/1851687793)