I hope from my last post, you got a fair idea about initial things, important to understand FSMO Roles (also called Operation Master). You know that Windows Active Directory structure is a multi-master operation. Means, in an AD, we generally have more than one master, called Domain Controllers. All DCs hold the database of Active Directory, containing details of all objects (User, Groups, OUs, Computers, Printers etc.) within the forest. If you are running your Infrastructure with one Forest, one Domain and one DC (I call it a “Self Assassinate Infrastructure” 
), FSMO roles doesn’t make any sense. But in 99.9% enterprises, you will not find such AD design.
Once you install your first Domain Controller, by default, it will hold all FSMO roles. But you can’t rely on single DC as it will create a single point of failure. To create a redundancy, you need to add few another (depending upon your organization size) Domain Controllers. Even if we have more than one DC, then we need/should split the roles to avoid any single point of failure.
Now the question is, what exactly FSMO is and why we need them? As we understood, in large environment, we may have many Domain Controllers, which means all of them will have complete control on the infrastructure. There are few things or responsibilities which are necessary to run our AD infrastructure in a proper way. These various responsibilities are basically FSMO roles. But since all of the DCs are masters, we need to decide which DC should hold what responsibilities. There are 5 specific responsibilities, need to be handled by one or multiple DCs. So they can be distributed in minimum 1 and maximum 5 Domain Controllers.
In short, FSMO Roles are set of functions (5 to be specific) which are necessary to run your Active Directory Infrastructure in a proper way. Always remember, only a Domain Controller can hold FSMO roles. Also these roles are distributed at Forest and Domain level separately.
In next topic, we will discuss the details of FSMO roles and it’s placement within the forest. Meanwhile, your feedback and questions are welcome.