>Vertx on the other hand, which it is built on and which itself builds on Netty

What does the highlighted part above mean?

I know what Netty is, but did not understand that phrase above,

Vertx uses Netty as the event loop handler for concurrent IO. Vertx adds a simplified asynchronous API on top of Netty (whose API can be pretty complicated), making backend services that need to handle large numbers of concurrent connections easy to implement. HTH