In simple terms, a quantum computer can simultaneously calculate every possibility in a set of arbitrary size, whereas a classical computer would have to calculate each possibility separately.

So for small operations that's not so helpful, but for massive ones it would be revolutionary. For example, consider calculating the 3d shape of a molecule with hundreds of atoms, or the interaction of several molecules with dozens of atoms. It is impractical to do this kind of calculation with the proper math due to the number of calculations/possibilities/interactions. Right now we use a 'rough' kind of calculation that is close enough but not close enough for many fields like medicine creation and protein folding. QC would make that task easy. It would also make AI training dramatically easier. Etc.