The Brief: Yee is an alternative to "Yes" or "Yeah" typically used in a celebratory manner.

DEEP DIVE
“Yee” is a word often said when someone is excited about something, synonymous with “Yes” and usually paired with an exclamation point.
For added emphasis, one might double-up, with an exclamatory “Yee Yee!” which can also be used interchangeably with “Yee Haw!” (depending on the context).
i literally do not wear any other item of clothing apart from this shirt but yee yee #beautysos pic.twitter.com/ihR6pN8kvU
— ➳bree loves calum (@motherlovedom) March 27, 2019
Oddly enough, though not too surprising considering the number of cartoons that have inspired memes (Scooby Doo’s Shaggy and The Simpsons‘ Ralph Wiggum, to name a few), “Yee” originally comes from the Italian language version of the animated movie Dinosaur Adventure, also called The Little Dinosaur.
the yee dinosaur is a bronchitis pic.twitter.com/PHJWPKwUj9
— Frank Gioia (@CrankThatFrank) November 6, 2017
The one-word phrase used by a green dinosaur inspired the popular “Yee Dinosaur” meme on YouTube.