Learn Italian: Oggi alla mensa c'è la pasta. - Today there is pasta at the canteen.

In Italian, you generally use definite articles like la when talking about a noun in a general sense or as a broad category. While English drops the word 'the' for general food items, Italian requires the article.

Why is it alla mensa instead of in mensa?

Both alla mensa (at the canteen) and in mensa (in the canteen) are correct and widely used. Alla is a combination of a (at/to) and the feminine article la. You will often hear in mensa as a set phrase without an article, but alla mensa is just as natural.

What exactly does c'è mean here?

C'è is the combination of the pronoun ci (there) and è (is), meaning 'there is'. If the canteen were serving something plural, you would use ci sono (there are) instead.

Does mensa refer to any kind of restaurant or cafe?

No, a mensa specifically refers to an institutional dining hall, like a school cafeteria or a company canteen. If you were getting food at a normal commercial cafe, you would use a word like bar or caffetteria.