What happens when rendering renderer and readyPromise in server.js Vue SSR?

Question

JavaScript

Hello.
I'm learning Vue SSR from the documentation and as a practice, I'm guided by the official example.
Code snippet from server.js

const isProd = process.env.NODE_ENV === 'production'

function createRenderer(bundle, options) {
  return createBundleRenderer(bundle, Object.assign(options, {
    
    basedir: resolve('./dist'),
    
    runInNewContext: false
  }))
}

let renderer
let readyPromise
const templatePath = resolve('./src/index.template.html')
if (isProd) {
  // In production: create server renderer using template and built server bundle.
  // The server bundle is generated by vue-ssr-webpack-plugin.
  const template = fs.readFileSync(templatePath, 'utf-8')
  const bundle = require('./dist/vue-ssr-server-bundle.json')
  // The client manifests are optional, but it allows the renderer
  // to automatically infer preload/prefetch links and directly add <script>
  // tags for any async chunks used during render, avoiding waterfall requests.
  const clientManifest = require('./dist/vue-ssr-client-manifest.json')
  renderer = createRenderer(bundle, {
    template,
    clientManifest
  })
} else {
  // In development: setup the dev server with watch and hot-reload,
  // and create a new renderer on bundle / index template update.
  readyPromise = require('./build/setup-dev-server')(
    app,
    templatePath,
    (bundle, options) => {
      renderer = createRenderer(bundle, options)
    }
  )
}

function render (req, res) {
  const s = Date.now()

  res.setHeader("Content-Type", "text/html")
  res.setHeader("Server", serverInfo)

  const handleError = err => {
    if (err.url) {
      res.redirect(err.url)
    } else if(err.code === 404) {
      res.status(404).send('404 | Page Not Found')
    } else {
      // Render Error Page or Redirect
      res.status(500).send('500 | Internal Server Error')
      console.error(`error during render : ${req.url}`)
      console.error(err.stack)
    }
  }

  const context = {
    title: 'Vue HN 2.0', // default title
    url: req.url
  }
  renderer.renderToString(context, (err, html) => {
    if (err) {
      return handleError(err)
    }
    res.send(html)
    if (!isProd) {
      console.log(`whole request: ${Date.now() - s}ms`)
    }
  })
}

app.get('*', isProd ? render : (req, res) => {
  readyPromise.then(() => render(req, res))
})

I understand that production and development builds are presented here, and it’s clear from the comments what this or that block of code does, but I don’t understand the situation with readyPromise (why it’s written this way and how it works). There is nothing in the documentation about readyPromise.
Thank you for your attention.