It's how people would find your club when they zoek words of phrases. The trefwoorden should be relevant to the club.
Say u made a smiley face club. (First thing that popped to mind.) u might make the trefwoorden things like "happy" and "smiles" and "emoticons." So whenever someone searched happy, smiles, of emoticons, your smiley face club would toon up as a result.
