Levarage Trading is a Big Question Mark To Me

I still don't understand how margin trading works. shorts? longs?

Could someone explain all this to me like I'm five? Every time I've asked, the descriptions were filled with words that I didn't understand/wasn't able to put in context due to lack of knowledge. I have accounts on all major exchanges, so an example through, say, poloniex's system would be ideal. Been trading for a while with good returns, but have never been bothered to do enough research on margin trading, how it works, etc.

Thanks in advance