what is the use of jwt token in node js code example

Example: jwt token example in node js

function authenticateToken(req, res, next) {
  // Gather the jwt access token from the request header
  const authHeader = req.headers['authorization']
  const token = authHeader && authHeader.split(' ')[1]
  if (token == null) return res.sendStatus(401) // if there isn't any token

  jwt.verify(token, process.env.ACCESS_TOKEN_SECRET as string, (err: any, user: any) => {
    console.log(err)
    if (err) return res.sendStatus(403)
    req.user = user
    next() // pass the execution off to whatever request the client intended
  })
}