Question from a curious flyer

Hey all. Sorry if this has been asked before already but I'm currently flying and just landed. And it just dawned on me that I've been wondering about this my whole life but still don't have an answer.

When the plane lands then the little flapper things come up on its wings, then it hits the tarmac and starts breaking....what is that loud sound that sounds like the engine reversing or something? Do y'all know what sound I'm talking about? Can someone explain it to me..it definitely seems to help the plane stop and brake properly. Thanks a bunch guys and gals.