It's how fast the pool finds blocks compared to its proportion of the global hashrate. For instance:
BTG block time: 10 minutes
Pool hashrate: 5 MH/s
Network hashrate: 50 MH/s
Pool has 10% of total hashrate, therefore should find blocks every 100 minutes on average
if it finds blocks with a 90 minutes average, luck = 90 minutes / 100 minutes = 0.9

Anything slightly below 1 is good, because indeed lucky (if you are superstitious), or because it has a slightly faster server connection or something.
Anything slightly above 1 is bad,
grossly above or below = fishy af

Luck is sometimes worded as "effort". Again, less effort to find blocks = good.

pool.gold ... well... just look effort from the last 20 blocks: 1.58, 1.06, 1.87, 6.14, 1.60, 5.90, 4.75, 0.22, 1.21, 4.17, 2.33, 9.59, 3.38, 2.49, 3.73, 0.42, 7.46, 4.42, 3.43, 0.69

Average luck, last 20 blocks... a whopping 3.32