Learn German: Der Ausdruck ist nämlich sehr wichtig. - The expression is very important, you see.

While nämlich can mean "namely" when listing things, it is very commonly used in everyday German to give a reason for a previous statement. Here it acts like "you see" or "because," explaining why the previous thought matters.

Why doesn't nämlich send the verb to the end of the sentence like weil (because) does?

Because nämlich is an adverb, not a subordinating conjunction. Words like weil introduce dependent clauses and move the verb to the very end. nämlich just sits inside a normal main clause and doesn't change the standard verb placement.

Can I put nämlich at the very beginning of the sentence to mean "Because..."?

No, you should avoid starting a sentence with nämlich. Unlike "because" in English, it almost always drops in right after the conjugated verb (in this case, ist) or slightly later in the middle of the sentence.

Does Ausdruck refer to a facial expression or a spoken phrase?

It can mean both! Just like the English word "expression," Ausdruck works for a phrase you say as well as a look on someone's face. It is directly related to the separable verb ausdrücken (to express).