Your application code will open a connection to a database to access or update data therein. "Failed DataBase Connects" is an exception where this connection to the database cannot be made. An example from one of my applications: org.springframework.jdbc.CannotGetJdbcConnectionException.
The reason for a failed database connection could be related to the connection-pool within your code (pool vs load), a resource issue on the database side, or even an issue in-between (e.g. network connectivity).
*I wish Dynatrace was better with monitoring connection-pools.
