It is a slang word that people use when saying yes. Its usually used when people see someone doing something well and they want to cheer them on and encourage them. So if you post a video doing a really cool dance I could reply "Yaaas! You are amazing"

Yaaasss is essentially a millennial way of saying "Yes I like that." It's an expression that has been co-opted from LGBT/Drag culture and has been made popular on shows like Broad City. It's just a fun way to say that you like something.