Learn Portuguese (Portugal): O teu filho é alto.

In European Portuguese, we almost always put a definite article (like o or a, meaning "the") before a possessive word. So instead of just saying teu filho, we say o teu filho (literally "the your son").

Does teu match the person I am talking to, or the son?

It matches the noun that is being owned or described, which is filho (son). Because filho is masculine and singular, we use the masculine singular possessive teu and the masculine article o. If you were talking about a daughter, both words would change to match her: a tua filha.

Can I use teu with anyone?

Teu is informal. You use it when talking to friends, family, or people you are close to. If you were speaking to someone formally (such as a stranger or a boss), you would use the formal "your" and say o seu filho.

Why do we use é instead of está for "is"?

We use é (from the verb ser) for permanent or inherent characteristics. Since being tall is a core physical trait rather than a passing mood or condition, we use é. If you wanted to say "your son is tired" (a temporary state), you would use está.