"Fellas" is an alternative way to spell the word "fellows". It's a slang word in American English for talking about a group of boys or men.
When you call a group of guys "fellas", it sounds like they're a relaxed, fun group of guys. You use it like this:
Chase is out watching a game with the fellas tonight.
Hey, what's up fellas?
The spelling of "fellas" is not officially correct, but it's somewhat common in casual writing like in text messages and social media posts.