Setting { extended: true } allows the bodyParser to accept json like data within the form data including nested objects. e.g. { person: { name: Adam } } sent using javascript rather than the name value pairs which traditional HTML form send. If you don't need that you can set the extended value to false. Not defining an extended option (i.e. using a default setting) is apparently deprecated and they seem to want you to decide whether you need nested options or plain name value pairs.
If you want to be able to parse form data for some routes and json data for others in your express server, you can use:
const express = require('express')
const app = express()
app.use(express.json()) // for json
app.use(express.urlencoded({ extended: true })) // for form data