First what is margin trading?
The easiest way to explain this is to compare with gambling! You are betting that price will go up or down and if you are right, you are going to make profit, otherwise you are going to make loss.
What does long position mean?
It means that you are betting that price will go up

But what does it actually mean?
It means that you are borrowing money from someone to buy contracts at current price. For example, you borrowed $1000 and you bought 1000 contracts of BTC at price of $10 000 per BTC. Let's say that BTC reached $11 000 and it is 10% gain, right?
Then you will close your positiong at $11 000 and you will return $10 000 to the owner, you will pay some fees to exchange and owner because you borrowed money from him/her and you will keep the rest. In this case it would be between $950 and $1000 ( it depends so it is not fixed ). So, I hope you got this!
What does short position mean?
It means that you are betting that price will go down

But what does it actually mean?
You are borrowing Bitcoin from someone and you are selling at current price hoping that you will buy it lower.
For example, you sold 1000 contracts at $10 000! BTW price drops to $9000 and it is drop of 10%. You decided to close position now! You will buy BTC at $9000 and you will return coins to the owner. You need to pay fees to the owner and to the exchange and rest is yours!
Now, my 1st advice for you!
If you have winning position, for example you entered long at 10 000 and current price is 10 300, put stop loss at 10 100 or 10 200. Do not risk to much. In oru manipulative market, everything is possible and secure your profit! If price jumps to 10500, then move your stop loss to 10300 or 10400 it is up to you. Secure your profit! Ride the the wave!