In the Netrunner games, "rezzing" a card is the act of revealing and paying for it when you're playing as the Corporation (in the new LCG anyway; I'm not familiar with the original CCG, but I believe it's the same).
The games take a great deal of inspiration from real computing terms for all of their "keywords" but this isn't one I'm familiar with. It must be short for something: "resurrect" is the best I can come up with, but doesn't really make literal sense since you're playing something for the first time. What's the origin of the term?