What it means
Shade is that slick little insult that slides in wearing a smile. It's not a full-on attack. It's a pointed comment, a sideways dig, a compliment with a tiny blade tucked inside. Petty with posture, basically.
Usage examples
"She said my haircut was brave, that is pure shade."
"He threw so much shade in that interview."
"When she said his outfit was very ambitious, that was shade and everybody knew it."
"Don't act sweet now, babe, you were throwing shade at me all brunch."
Where it comes from
The term comes from Black and Latino drag ballroom culture in the US. It was popularized wider through queer slang, especially via Paris Is Burning, where shade meant a stylish, indirect form of disrespect. Not loud beef, more a cool, cutting read that lets you keep your hands clean.