It's an expression - a phrasal verb Often used to refer to something that has been done or an occurence which changes the outlook of the thing being discussed E. G. "The tennis tournament was quite competitive until Djokavic convincingly defeated his opponent - this set the tone of the competition" Hope I explained that ok :) Shukran

You didn't give us any context. In creative writing or visual arts, it could mean to establish a mood. In teaching or training, it could mean to make the rules and expectations understood. In audio or video reproduction, it could mean adjusting the playback settings.