Learn Russian: Завтра мы увидимся на концерте. - Tomorrow we will see each other at the concert.

The -ся ending makes the verb reciprocal, meaning the action goes both ways. Without it, мы увидим means "we will see" (someone or something else). Adding -ся turns it into "we will see each other."

This lesson is about using с (with) + Instrumental case, so why isn't there a с in this sentence?

Because the subject is мы (we). When you use a plural subject like "we" or "they" with a reciprocal verb, both parties are already included in the subject, so you don't need с. If the sentence was "I will see you," then you would need it: Я увижусь с тобой.

Is увидимся future tense? It looks like it's conjugated in the present tense.

Yes, it is future tense! The verb увидеться is perfective (it focuses on a completed result). When you take a perfective verb and conjugate it with present-tense endings, it automatically creates the simple future tense.

Why do we use на концерте instead of в концерте for "at the concert"?

In Russian, you use the preposition на rather than в for public events, performances, and gatherings. Since a concert is an event rather than a physical building, you are на концерте.