Learn Dutch: Jouw haar is mooi. - Your hair is beautiful.

Both jouw and je can mean "your". Jouw is the stressed form. You use it when you want to put special emphasis on the owner, like saying "YOUR hair is beautiful" (as opposed to someone else's). If you don't need that emphasis, you can just say Je haar is mooi.

Is jouw a reflexive pronoun like the ones we are learning in this lesson?

No, jouw is a possessive pronoun (meaning "your"). This sentence is a great reminder of the difference: you use possessive pronouns to show ownership of a noun (jouw haar), but you use reflexive pronouns (like me, zich, or the reflexive version of je) for actions someone does to themselves (like "you wash yourself"). While je can be used as both a possessive and a reflexive pronoun, jouw is strictly for possession.

I thought haar meant "her". Does it also mean "hair"?

Yes! In Dutch, haar can be the object pronoun "her" (ik zie haar - I see her), the possessive pronoun "her" (haar boek - her book), or the noun "hair". Here, because it comes right after the possessive pronoun jouw (your), it has to be the noun "hair".

Why is the adjective mooi instead of mooie here?