Learn Swedish: Visningen börjar antingen klockan två eller tre. - The viewing starts either at two or three o'clock.

This pair means "either ... or". You use it exactly like its English equivalent to present two choices. Here, antingen introduces the first choice (klockan två) and eller introduces the second choice (tre).

Can I move antingen to the beginning of the sentence?

Yes, but remember that Swedish is a "verb-second" language. If you start with the first choice, the verb must immediately follow it. The sentence would then become: Antingen börjar visningen klockan två eller tre.

Where does the word visningen come from?

It is the definite form (the "the" form) of en visning, which means a viewing, a showing, or a guided tour. It is built from the verb att visa, which means "to show".

Why do we say klockan två eller tre instead of just två eller tre?

Klockan literally means "the clock" and is used just like "o'clock" in English. While you can sometimes drop it in casual speech, adding klockan before the first number is the most natural way to state a time. Notice that you do not need to repeat it before tre.