Learn Swedish: Komikern, vars skämt var jätteroliga, vann tävlingen. - The comedian, whose jokes were really funny, won the competition.

The prefix jätte- literally means "giant," but it acts as a very common intensifier meaning "really," "super," or "extremely." It can be attached to almost any adjective, both positive (jätteroliga) and negative (like jättedålig for "really bad").

Why does jätteroliga end in an -a?

Adding an intensifier prefix like jätte- doesn't change the normal rules for adjective agreement. Because it describes skämt (jokes), which is plural in this context, the base adjective rolig still requires the plural -a ending.

How do we know that skämt is plural in this sentence?

Ett skämt (a joke) is an ett-word ending in a consonant, so its plural form is identical: flera skämt (several jokes). You can tell it's plural here because the adjective describing it, jätteroliga, has the plural -a ending. If it were just one joke, the sentence would say vars skämt var jätteroligt.

Does the relative pronoun vars (whose) change depending on the gender or number of the noun?