Pool luck explains how many shares the pool needed to find a specific block in comparison to the average number of shares needed for finding a block based on the current network difficulty.
If luck is above 100%, it means that the pool needed fewer shares than expected for the given difficulty. If the luck is below 100%, more shares were needed than expected.
E.g. if the luck is 200%, the pool found two times more blocks in the given time period than average.
Please, keep in mind that luck only describes the history but says nothing about the future blocks. Finding a block is fully independent from historical events. That is why joining the pool when the luck is high and disconnecting when the luck is low does not make any sense. In the long run, the luck averages out to 100%.
We have also created a detailed article on this topic that explains all the basics in broader context.