This is where the expression 'throw shade' comes from
The term "throwing shade" comes from black and Latino gay communities. The term's first significant step into straight culture was in the 1990 documentary about young, black and Latino drag queens in New York City, 'Paris is Burning.' Throughout, the central characters explain their culture and guide you through the underground world of parties and drag balls.
In one scene, one queen named Dorian Carey explains what "shade" is.
"Shade is, I don't have to tell you you're ugly, because you know you're ugly," she says.
Now, when someone insults you directly, that's called a "read." For example, if I were to tell you that your glasses are ugly. Point blank. That's a read. Reads can be long or they can be short.
"Shade" comes from reading, as Cary explains.
If I were to say in a terribly condescending voice, "Oh honey, I'm so glad you saved up to buy those glasses." That's blatant shade. I didn't insult the glasses, or you, directly. It's implied by my voice and the context of what I said. You know they're ugly.
Sometimes people don't get that they're being "shaded" - this is always sad.
To "throw shade" simply means you've said something shady to someone. If you want an example of someone who throws perfect shade in every way, that would be pop star Rihanna. She can shade you with her eyes.
Watch Dorian Carey explain it in Paris is Burning below: