body-parser is an NPM package that parses incoming request bodies in a middleware before your handlers, available under the req.body property.
app.use(bp.json()) looks at requests where the Content-Type: application/json header is present and transforms the text-based JSON input into JS-accessible variables under req.body. app.use(bp.urlencoded({extended: true}) does the same for URL-encoded requests. the extended: true precises that the req.body object will contain values of any type instead of just strings.