Learn Russian: Он всегда мечтает о тебе.

The preposition о (meaning "about") requires the following word to be in the Prepositional case. тебе is the Prepositional case form of the informal "you" (ты).

Why use о instead of об in this sentence?

We use о before words that start with a consonant, like the "t" in тебе. You will see it change to об when the next word starts with a vowel (like об Иване - about Ivan) to make it flow better when speaking.

Does мечтает mean he is having a dream while sleeping?

No, the verb мечтать means to daydream, wish, or romanticize about something or someone. If you want to talk about the literal dreams you have while asleep, Russian uses completely different words. Here, it means he is fondly thinking or daydreaming about you.

Why does the в in всегда sound like an English "f"?

This happens because of a rule where consonants blend together. The letter в is a "voiced" sound, but it sits right next to с, which is "voiceless." The с forces the в to drop its voice, making it sound like an "f". It is pronounced like "fseg-DA".