PRNG stands for Pseudo-Random Number Generation. Essentially it's a method of random number generation that more evenly distributes the chances of success. In the context of DOTA or LEAGUE, it means that there's a far smaller chance of being "streaky" (having a ton of crits in a row, or none at all for a long period of time). If you have the time, take a look at the gamepedia wiki entry on

PRNG (beware its dota2 gamepedia XD). The basics though work like this: PA ADC's actual chance to crit on her YOUR first attack is 3.222% (not 15% if you happen to get ghostblade as your first item Kappa). However, each subsequent attack she you do after that increases her your chance to crit by another 3.222%. So her your 2nd attack will have a 6.444% chance to crit, her 3rd attack will have a 9.666% chance to crit, and so on until the crit procs. Once the crit procs, the percent is reset back to 3.222%. This averages out to her you critting 15% of the time over many hits, but each individual attack will have a far different chance to crit on its own. The quick ones of you will notice that this system can be gamed. If you count your attacks since your last crit, you can compute the probability of a crit on your next attack. The returns aren't that great though - a 3% increase each attack isnt much.