The term “luck” refers to the probability of successfully mining a block. The ratio between the number of daily, actually mined blocks and the theoretical number of existing blocks is known as the “luck value”.
For example, according to the ratio between a mining pool’s hashrate to the network’s total hashrate, let’s assume theoretically that 10 blocks will be generated per day. If 12 blocks were successfully mined by a pool in a day, the luck value would be 120%; if it is 8 blocks, the luck value would be 80%.
Luck value generally affects the rewards with the payout schemes of PPLNS. With FPPS, the whole network's luck value affects the rewards from transaction fees (Bonus part).