What is the schema master?

Active Directory is just a database.You create things called objects that go into your database (users, computers, printers, etc). Let's look at user objects. Each user object has its own unique information called attributes (i.e. name, address, phone number, position, etc.)So I have a user- Bob. When I create my user in AD, I'm creating a user object. The things that describe the user object (Bob's name, address, phone number, title, etc) are the attributes that are collected about the user object.Same thing for computers, printers, etc.All of these different objects and the attributes that you can collect about the objects make up the Schema.The Schema is the blueprint for your database.If you decide later that you need to add more attributes to an object (i.e. you need to collect every user's social security number in the US) you have to modify your Schema.The only DC that can modify the Schema is the Schema Master. Also, the user has to be a Schema Admin.The Schema Master is one of the five FSMO roles (Schema Master, Domain Naming Master, RID Master, Network Infrastructure Master, and PDC Emulator). There is only one DC in the entire forest that holds the Schema Master role. By default, it is located on the DC that was the first one created in your AD forest.

The schema is the Active Directory component that defines all the objects and attributes that the directory service uses to store data.The DC responsible to for performing updates to directory schema is termed as Schema Master.Schema Master is required While the time of operating system migration, installing new Exchange version and any other application which requires extending the schema, and so it is not a critical FSMO which will impact the end-user immediately as PDC. 

Schema Master or FSMO role owner is the domain controller responsible for doing updates to the directory schema. Each forest will have only one Schema Master.