It’s a prefix that is very common which means ’very’. So instead of saying mycket/väldigt bra for ’very good’ you say jättebra. You can add it to virtually all adjectives to intensify it, where English would have ’very’. It literally means ’giant’ but that meaning is sort of lost, so you can also say jätteliten (very small) even though that might seem like an oxymoron.
It's not quite "deejettebra", As a non-native English speaker and as a non-native Swedish speaker, I would nevertheless say that I sounds like "Djärettebra" and är here melds together with "Det" which is "Diye" so that I becomes Diyerettebra or "Dijärettebra". I listened to the Swedish singer Veronica Maggio, and her music kind of helped me to figure out how we should pronounce a vowel following an "är", She sang like "Den första är altid gråtis" and I couldn't understand why it's "Den först-ä-rialtid, seems like they combine two vowels when say a vowel(in a word) + är, I hope it helps and what I say is not wrong because it's just my own listening experience