Material UI slots

I am trying to wrap my head up around the concept of slots in Material UI, but can't find a way to understand them, as I could not find any reference in the official documentation.

I see a lot of objects namedslots in the Material UI core component library, can someone explain what are those and how are they used?

For instance in this function:

const useUtilityClasses = (ownerState) => {
  const { classes } = ownerState;

  const slots = {
    root: ['root'],
  };

  return composeClasses(slots, getCardUtilityClass, classes);
};