Learn German: Es sitzen heute viele Studenten im Hörsaal. - Today there are many students sitting in the lecture hall.

Es is acting as a placeholder or "dummy subject" just to fill the first position in the sentence. The actual subject is the plural viele Studenten (many students), so the verb must be plural to match them. This is a common way in German to express "There are [people] doing [action]."

Can I put the subject at the beginning instead?

Yes! You could simply say Viele Studenten sitzen heute im Hörsaal. If you do that, the placeholder es disappears entirely. You only use this type of es at the very beginning of a sentence to delay the real subject.

What are the parts of the compound noun Hörsaal?

It is made from the stem of the verb hören (to hear or listen) and der Saal (hall or large room). It literally translates to "listening hall," which perfectly describes a university lecture hall.

Why does the sentence use im rather than just in?

im is a mandatory contraction of in dem. Because the students are simply sitting in one place (a state of rest rather than moving to a new location), the two-way preposition in requires the dative case here. Hörsaal is masculine (der Hörsaal), so the dative article is dem.