It’s simple. This option, available in SQL Server 2008 standard and forward, will prevent long running queries based on run time measured in seconds. If I specify a value of 180 the query governor will not allow any execution of a query that it estimates will exceed that value. Notice it says ESTIMATES which means it will be based on optimizer estimates and not ACTUAL run times. It does NOT KILL an actively running query after designated amount of time. There is no worries for rollback scenarios or partial data.
This is totally different from Resource Governor, which can throttle resources like CPU and I/O. I’ve never turned on query governor and I’m not sure I ever would.