Learn French: Il vaut mieux hydrater la peau tous les jours. - It is better to moisturize the skin every day.

It means "it is better," and vaut comes from the irregular verb valoir (to be worth). Literally, the phrase translates to "it is worth better." It is an impersonal expression, which means the il (it) is just a grammatical placeholder and doesn't refer to a specific person or thing.

Why isn't there a de before hydrater? I usually see c'est mieux de + infinitive.

The structure il vaut mieux is special because it is always followed directly by an infinitive verb, without any preposition. While you do need a preposition for c'est mieux de faire (it's better to do), il vaut mieux simply skips the de.

As an English speaker, I would say "moisturize your skin". Could I use ta peau here?

You could, but French strongly prefers using definite articles (le, la, les) for body parts instead of possessive adjectives (ton, ta, tes). This is especially true for general statements or when the "owner" of the body part is obvious from context. Here, la peau is the most natural choice.

Why is it tous les jours instead of tout les jours?