1. What is Short?
To understand this first look at Long, Long is when you have bought a stock, when you think that the stock would move upward. While Short is when you have sold a stock(even if you dont own it), when you think that the stock would move downward.
You cannot carry stock short beyond intraday, but can carry short position in futures and options.
2. What is Position?
When you buy and sell a stock you are having a 'position' in those stocks. Either long or short.