You can think of a role as a type of container for holding one or more logins, users, or other roles, similar to how a Windows group can hold multiple individual and group accounts. This can make managing multiple principals easier when those principals require the same type of access to SQL Server. You can configure each role with permissions to specific resources, adding or removing logins and users from these roles as needed.
SQL Server supports three types of roles: server, database, and application. Server roles share the same scope as logins, which means they operate at the server level and pertain to the database engine as a whole. As a result, you can add only server-level principals to the roles, and you can configure the roles with permissions only to server-level securables, not database-level securables.
These help form the foundation of a secure instance, so it’s vital to understand these concepts.