Learn Swedish: Recensenten gäspade ofta, vilket betyder att föreställningen var usel. - The critic yawned often, which means that the performance was awful.

In Swedish, you use vilket when "which" refers back to an entire situation or clause. Here, it refers to the whole fact that the critic yawned often. If "which" referred to a specific noun, you would use som.

Could I use något som instead of vilket in this sentence?

Yes, you could say ...något som betyder att... which translates to "something that means that". Both vilket and något som work well for referring back to a complete thought. Vilket is just slightly more formal and very common in writing.

Where does the word föreställningen come from?

It is the definite form of en föreställning, meaning a performance or a show. It is built from the verb att föreställa (to represent or depict) plus the suffix -ning, which turns verbs into nouns just like "-tion" or "-ing" in English.

What kind of word is usel?

Usel is a strong adjective meaning awful, lousy, or miserable. It is a great alternative to simply saying mycket dålig. Because it describes föreställningen (an en-word), it takes the basic en-form here. For an ett-word it would be uselt, and the plural is usla.