什么是嵌套路由？如何定义嵌套路由？

import React from 'react';
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';
const HomePage = () => <h1>Home Page</h1>;
const AboutPage = () => <h1>About Page</h1>;
const ContactPage = () => <h1>Contact Page</h1>;
const InnerPage = () => <h1>Inner Page</h1>;
const App = () => (
  <Router>
    <Switch>
      <Route exact path="/" component={HomePage} />
      <Route path="/about" component={AboutPage} />
      <Route path="/contact" component={ContactPage} />
      <Route path="/inner" render={() => (
        <InnerPage />
      )} />
    </Switch>
  </Router>
);