Content clauses are subordinate clauses that may or may not begin with a conjunction. They serve as the object of the independent clause, so in German they are also known as object clauses. Like all subordinate clauses, they are dependent upon the independent clause or another dependent clause to which they refer and cannot stand on their own. You can easily recognize an object clause (and also subject clauses, which are content clauses that serve as the subject of the main clause) by the fact that the superordinate clause also cannot stand on its own; it requires the content (subordinate) clause to be complete.

Content clauses frequently begin with the conjunction "dass". They can also begin with question words, such as "wo", "wie", "was", "wann", "womit" etc. These clauses are called interrogative content clauses. In German, they're also known as "W-Sätze" (W object clauses) because all the German question words begin with the letter "w".

Examples:
Wir wissen, wo wir Ostrowski finden.
Das Orakel hat ihm gesagt, was er tun muss. (Beispiel: Das Orakel hat es ihm gesagt.)
Sie sehen ja, wie wir hier wohnen. (Beispiel: Sie sehen es ja.)