Learn Swedish: Om ugnen är kall, måste man vänta. - If the oven is cold, one must wait.

This is the V2 (verb-second) rule in action! The entire first part of the sentence, Om ugnen är kall (If the oven is cold), counts as the first building block. Because the verb must always come second in a main clause, the verb måste has to come right after the comma, forcing it to swap places with the subject man.

What exactly does man mean here? Is it the word for a man?

In this context, man is an impersonal pronoun. It is used just like "one" or a general "you" in English. So måste man vänta means "one must wait" or "you have to wait" in a general sense. The noun for a male person is also man, but here it acts as a pronoun.

Why is there no att (to) before the verb vänta?

Because måste (must) is a modal verb. Just like in English, Swedish modal verbs are followed directly by the infinitive form of the next verb without using att. We say "one must wait", never "one must to wait".

Why does the sentence use ugnen instead of just ugn?

Ugnen is the definite form, meaning "the oven". In Swedish, you add the suffix -en to the end of an en-word like ugn to make it definite, rather than putting a separate word for "the" in front of it.