The dbo User Account

The dbo User Account. The dbo, or database owner, is a user account that has implied permissions to perform all activities in the database. Members of the sysadmin fixed server role are automatically mapped to dbo. dbo is also the name of a schema, as discussed in Ownership and User-Schema Separation in SQL Server.

Server and Database Roles in SQL Server - MSDN - Microsoft

https://msdn.microsoft.com/en-us/library/bb669065(v=vs.110).aspx