For which version? For SQL 2016, you shouldn't be using this, as the new cardinality estimator is on.

For 2014, this should help: https://support.microsoft.com/en-us/help/974006/sql-server-query-optimizer-hotfixtrace-flag-4199-servicing-model

Essentially, if you enable this with a database in compat level 120, this turns on the new optimizer for the database.