Pay Per Last N Shares refers to a reward system used by some mining pools, where the miner is paid for the last N shares he provided either time based or as a percentage of total shares supplied by all miners active on the pool.
Pay Per Last N Shares is open to variance and luck, as the pool may either have relatively steady amounts of blocks found, or either get them in batches or not at all for a period. If you mine consistently on PPLNS pool, the earning will remain fairly consistent, and fluctuations will appear only as new miners join or abandon the pool.
PPLNS is an inherently riskier method of reward for miners, due to the fluctuation of earnings over time.
Lastly keep in mind that a lot of PPLNS enabled pools are being marginalized to become profitable for the pool operator, and have a higher payout threshold which in turn makes cashing out slower.