it's were the cam shaft sprockets can be rotated in relation to the cams when the fastening bolt is losened there fore they are not fixed in place
this is done in manufacturers case for more acurate engine timing they are normaly allowed to turn only one tooth or so either way
in tuning you can buy vernier pulleys to replace fixed pulleys on a regular car in order to drastically change you cam timing to raise or lower your power band and adjust other settings such as valve overlap