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