Automatic failover is supported only in database mirroring sessions running with a witness in high-safety mode (high-safety mode with automatic failover).

In high-safety mode with automatic failover, once the database is synchronized, if the principal database becomes unavailable, an automatic failover occurs.

An automatic failover causes the mirror server to take over the role of principal server and bring its copy of the database online as the principal database. Requiring that the database be synchronized prevents data loss during failover, because every transaction committed on the principal database is also committed on the mirror database.

For automatic failover to improve reliability, the mirror and principal databases must reside on different computers.

Hardware & Software requirements
https://docs.microsoft.com/en-us/sql/...

Data Mirroring (High safety and High Performance mode) :    • Database Mirroring in SQL Server || D...