A long position is when you purchase the shares outright and hold them. A short position is when you borrow the shares and sell them, planning to reimburse the lender of those shares later with shares that you purchase...hopefully at a lower price...therefore netting a profit. Make sense?
If you buy long you make a profit on the increase of share price, and the potential for loss is limited. If you sell short, you make a profit on the decline of share price, and the potential for loss is unlimited. (You lose money when the share price goes up).