what is server roles in sql server?

It is used to manage permission in server. These are kind of security groups which be assigned to a user in sqlserver. You can also create user definedserver roles from 2012 onwards. Server roles arebulkadmin, dbcreator, diskadmin, processadmin, securityadmin, serveradmin, setupadmin, sysadmin etc.

 

Moreover, go through below link:

 

https://msdn.microsoft.com/en-us/library/ms188659.aspx