Grammar Help

To preface, I'm teaching myself German so I don't have access to an instructor.

I was reading something in German online a few days ago, and I would appreciate help identifying what it is called, and how it is used. Where an object is stated in one clause, but then the article is substituted for it in the dependent clause. Something like "Ich sehe den Mann, den sehr alt ist." I was able to figure out they were referencing "den Mann" but I'm not sure what grammar rule that would be called or the rules to follow for it. Any help is appreciated.