https://docs.microsoft.com/en-us/sql/relational-databases/databases/system-databases
SQL Server includes the following system databases.
1.master
Records all the system-level information for an instance of SQL Server.
2.model
Is used as the template for all databases created on the instance of SQL Server.
Modifications made to the model database, such as database size, collation, recovery model, and other database options, are applied to any databases created afterward.
3.msdb
Is used by SQL Server Agent for scheduling alerts and jobs.
4.tempdb
Is a workspace for holding temporary objects or intermediate result sets.
5.Resource
Is a read-only database that contains system objects that are included with SQL Server. System objects are physically persisted in the Resourcedatabase, but they logically appear in the sys schema of every database.