Margin trading actually means trading by using a loan from the market maker or trading platform you are using. This loan will provide you with a leverage and allow you to control MUCH bigger amount of coins than you could with your own resources.
Usually, there is some interest charged for the loan, or you get a bit worse pricing or higher fees per trade ... the lending party has to make money somehow, right? :)
The possible gains are way higher, but so are possible loses, so do not engage in margin trading unless you really know what you are doing.
There is a lot of controversy surrounding large margin trading platform, like Bitmex, they are being accused of market manipulation and blocking trading activity at times, without any clear reason.