TLDR

Statikk shiv stacks on one champion. That means that when a champion with multiple statikk shivs, every third attack, all statikk shivs will proc individually. This results in massive early game damage.

Full explanation

Statikk shiv is an item that causes a champion's third auto attack to deal 100 splash magic damage. Building multiple statikk shivs on a champion will result in this third auto attack procc'ing multiple statikk shiv passives.