1/1 = 100% Chance, 1/2 = 50% Chance, 1/5 = 20% Chance, 1/10 = 10% Chance and so on.
Some people get "Lucky" when an item drops. Something could be 1/1,000,000 And get it in the very first drop.
Once it drops I think the item recycles. So you have to get that luck again to be able to obtain it...