Pool luck makes clear the number of shares the pool needs to uncover a specific block compared to the average number of shares necessary for locating a block based on the existing level of network difficulty. If luck is over 100%, this means that the pool required fewer shares than anticipated for the given level of difficulty. If the luck is under 100%, more shares than was expected were required. 

Example: If luck is 200%, the pool discovered two times more blocks over a specific time period than what was average. 

Do note that luck only relates to mining history and does not predict the outcome of future blocks. Locating a new block is an endeavor that is completely separate from historical events. For this reason, joining a mining pool when the luck percentage is high and disconnecting when the the percentage is low doesn’t make sense. Ultimately, it averages out at 100%.