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It's shorthand on Twitter to refer to a company: https://twitter.com/search?q=%24AAPL&src=typd

In this case, it's the same number of characters as the longhand.

But $AAPL is clearer, because "Apple" may refer to the record company, or the fruit!

(I'm stretching, I know.)

It's the stock symbol.

With the $ sign?

he's trying to belittle the company