Finally some discussion which is pool related, so I will try to explain ...
We know (I hope) that usually block is found after exactly difficulty shares are submitted, which is equal to 100% luck, but not all rounds - CDF shows how many rounds (in percent) are completed in the current number of shares submitted.
Example as of right now:
Current round duration: 3:54:31
Current shares CDF: 51.75 %
Current Bitcoin block, difficulty: 327538, 35985640265
Shares contributed in current round: 26222487684
We have submitted (since the round start) 26222487684, with the current difficulty equal to 35985640265, so we have submitted 26222487684/35985640265=0.72869309788283129217792729468892
In short 0.73, then we look at the graph and it shows CDF being 52%, which means in 52 of 100 rounds we would have found a block by now.
Example from the previous round:
Duration: 10:25:16
Total Shares: 69928582694
69928582694/35985640265=1.9432357512341735737628677148118
or 1.94 = CDF of 86%, which means that 86% of our rounds are (should be) shorter than that