“Usseewa” (うっせぇわ) is a rude, slurred way of saying “shut up!” — and Ado, only 17 when she released it in 2020, sings it at everyone who lectures her: the bosses with their manners, the people who tell you what's normal and correct. She's been polite, she's followed the rules, and she's had enough.
The song became a phenomenon in Japan, a little scandalous for its language and hugely popular with young people tired of social pressure — especially office culture.
It's C1: very fast, with deliberately rough pronunciation. But the key words are clear and very useful for understanding modern Japan: common sense, society, correct, normal.