S&R are points where price will bounce off or have a hard time going through. They're also significant levels of interest to traders, so worth keeping an eye on.
They're also good for timing trades - if you want to buy, then you're looking for a good support level to time your entry. The opposite is true if you're looking to sell.
Buying and selling both carry risk. Neither is better than the other in a Forex environment, nor is selling borrowing. In FX, what you buy and sell is currency and given its dual nature, you are simultaneously long and short either in a given pair.
For example, if you are long Eur/Usd, then you either bought Eur, or sold Usd. If you are short the pair, then you sold Eur, or bought Usd.
What you are thinking of is short-selling, which is unique to stocks. In this case, you are effectively borrowing stock at a certain price and giving it back at a future price. The difference between the two represents your profit/loss.
Hope it helps. 🙂