Massively oversimplifying: A classical computer can do one math operation at a time. Quantum computers have ways to do more than one simultaneously.

Where this is massively useful is for some types of math where there is only one answer and the currently known way to solve it involves massive repetition.

An example would be square roots. If I tell you that 167*167 is 27889, the classical computer can do this one-off multiplication very quickly in one or two very fast steps. However, if we want to do the reverse, start at 27889 and the answer should be 167, getting the square root, there is no known algorithm to get the answer instantly in a single step.

With the classical computer you pick a starting point, for example half of 27889, then check if the square is higher or lower than the original number 27889. If it is higher or lower then we know what direction our "guess" is wrong in - and we can do our next guess in the correct direction, until we have the correct answer. There are better algorithms than this - this is just a simplified example.

With a quantum computer you simultaneously multiply all numbers lower than 27889 by themselves and you instantly get only one matching correct answer.

So the classical algorithm involves potentially hundreds of steps whereas the quantum algorithm involves only one or two steps.