Short sellers basically bet against troubled stocks. /r/WallStreetBets called them on it by pumping some of those stocks up, costing short sellers billions.
But why? Who will profit?
Short sellers get slaughtered, /r/wallstreetbet pumpers sell before the inevitable crash and profit.
Anyone who bought at the right time and sells at the right time. Certainly a lot of people on /r/WSB.
Those betting against the short sellers profit. The short sellers are obliged to buy stock to cover their short positions which are suddenly quite expensive. Their buying then further creates demand pushing the price up even more - a short squeeze.
The "but why" is essentially "because fuck those guys". I'm not sure there was any more than that.
As for who will profit, probably not the people who were shorting GME, but at this point it's impossible to say.
Some trades are strategy and some are tactics. This is the latter.
WSB realized the short sellers had bought more shares short than there were shares available to buy, therefore creating a squeeze and inevitable price increase. They then encouraged everyone to buy shares to screw the shorts and make money.