What does it mean when someone " shorts" or "shorting"a stock, or selling a stock short mean?

Answer 1

A short is basically the act of borrowing a stock and selling. The stock is later covered, re-bought hopefully at a lower level, in order to return the stock. It is basically the play to profit from a stock that is going down.

There is limited upside to shorting a stock of 100% of the stock value. There is unlimited downside to a short, where the stock can potentially go up infinitely.

Answer 2

Shortin stock means selling stocks with the expectations that the price is going to go down. The margin requirement for shorting is higher than long buys. It is aroung 45% of your order while it is only 35% for your long buys.
What the broker does is he will sell the stocks you short from his inventory or from borrowed from fellow brokers. Put that money into your account. If the order is to close at some point then as the price goes down to that level he will buy the stock back and put it back in his inventory or give it to the broker from whom he borrowed, at lower price. He will give you the difference.
If the price goes up instead of down, your escape route is a stop loss order where the broker will cover at prices affordable by you above the price at which you executed the short sale order. So usually when you short sell you will give a close price and cover price thus preventing unlimited loss and maximising returns.