query.h wrote:"Fire" -> Looks for all files which contain the word "Fire"
"Fire OR water" -> Looks for files which contain either "Fire" or "Water". The capitalization of the words doesn't matter as that will be ignored internally. However, OR and AND have to be in upper case.

"artist:Coldplay" -> Look for any files with the artist "Coldplay"

"artist:(Coldplay OR Maroon5) power" -> Look for files with the artist Coldplay or Maroon5 and the word "power"
"artist:'Noah and the Whale'" -> Look for files with the artist "Noah and the Whale"
"type:Audio title:Fix" -> Look for Audio files which contains the title "Fix" in its title.

The Query Parser recognizes a large number of properties. These property names can be looked up in KFileMetaData::Property::Property. The type of the file can mentioned with the property 'type' or 'kind'.