It is essentially a form of wealth management for FOREX. If you were to sign up for a Slave account, you can select a Master account to follow. That means that your account will automatically replicate the trading strategy of the Master account in real time. Your account will execute the same trades as the Master account, but scaled for the total capital in your account. When the Master profits, the Slave profits. Slaves profiting is in the best interest of the Master.
You can view the performance and historical trades of Master accounts to decide which to, "follow" and you can unpair your account with a Master at any time.
You will NOT be able to see the exact trades happening in real time because if that were allowed people could just report real-time trades of top traders to other users without benefiting the Master, essentially removing all of the incentive for successful traders to use Master accounts. However, you can view the trades after a period of time that I believe will be determined in the agreement set by the Master in this case.
Basically, you are leeching off of the strategy of a Master and in return giving them a portion of any profits. Your account will trade as a clone of the Master account while paired, but scaled to fit your total available trading capital. All of this is automated and the Master doesn't have access to your account.