Q: what is back to my mac?

iCloud- Setting up and using Back to My Mac with an 802.11n-based AirPort base station, or Time Capsule

Mac OS X 10.7 Help- Use Back to My Mac

Mac OS X 10.7 Help- Share your computer’s screen using Back to My Mac

Supported router devices for Back to My Mac

Using and troubleshooting Back to My Mac with your iCloud account on OS X Lion

Well, I thought you might figure that out.

"Back to My Mac provides remote access to your Mac from another Mac anywhere on the Internet."

http://en.wikipedia.org/wiki/Back_to_My_Mac

It is basically a way for a Mac owner to access another Mac sharing the same Apple ID over the internet. The screen can be shared, the remote Mac can be controlled and files can be transferred to and fro.

Sorry for even trying to help. The information was there for you to find. You were to lazy to look. So I gave you the definition. What's your problem?