Can anyone explain what does middleware does in this example

export default function clientMiddleware(client) {
return ({dispatch, getState}) => {
return next => action => {
if (typeof action === 'function') {
return action(dispatch, getState);
}
const { promise, types, ...rest } = action;
if (!promise) {
return next(action);
}
const [REQUEST, SUCCESS, FAILURE] = types;
next({...rest, type: REQUEST});
const actionPromise = promise(client);
console.log('client ');
console.log(client);
actionPromise.then(
(result) => next({...rest, result, type: SUCCESS}),
(error) => next({...rest, error, type: FAILURE})
).catch((error)=> {
console.error('MIDDLEWARE ERROR:', error);
next({...rest, error, type: FAILURE});
});
return actionPromise;
};
};
}