"Stacking" is basically accomplishing the requirements to boost their passive by 1. Think of it like gaining 1 extra AP or AD by killing minions. "Stacking" is basically getting as many minion kills as possible to gain maximum bonus AD/AP, which means higher damage.
Stacking in RPGs refers to stat stackng (See min-maxing). Stat stacking is when players will load up on stats that are the most beneficial for their class or their role in the game, giving them huge amounts of one or two stats and very low amounts in the other.