Learn Swedish: Vill du hellre komma imorgon? - Would you rather come tomorrow?

You're right that vill usually means "want". However, when you combine it with the adverb hellre (rather), the phrase vill ... hellre translates smoothly to "would rather" or "prefer to". You are literally asking "Do you want more gladly to come tomorrow?".

What kind of word is hellre?

Hellre is the comparative form of the adverb gärna (gladly). If you do something gärna, you do it gladly. If you do it hellre, you'd "rather" do it. The superlative form is helst (most preferably).

Why isn't there an att (to) before the verb komma?

Because vill is a modal (helper) verb. In Swedish, helper verbs like vill (want to), kan (can), and måste (must) are followed directly by the infinitive form of the main verb without using att.

Could I put hellre at the end of the sentence, like "Vill du komma imorgon hellre?"

No, that would sound unnatural. Hellre acts as a sentence adverb here. In a question, the standard word order is Verb (Vill) + Subject (du) + Sentence Adverb (hellre), followed by the rest of the sentence.