First of all, if you never saw how to put the Heroic Mode, it's kinda easy. When you Rgith click on the picture of you on the left corner of the screen you have the option to put Heroic Mode or not. But what's the difference?
The difference appear only when you enter an instance. The difference is that the instance become like a lvl 70 instances for epic drops. That's mean that the boss will drop Epic items.. even if normally it don't do that. Of course the instance become more difficult. The mobz will be stronger or more than normally. If you want to see the drops of an instances in Heroic Mode, you only have to go on Wowhead, search the name of your instance and look in the Heroic Drops.
Any questons?
How to put Heroic Mode:
How to see the drops in Heroic Mode: