The phrase "mod system" refers to the way modifications or changes are made to something. It's a system or process that allows you to customize or alter something according to your preferences or needs. Full definition
Well, we'll throw in modest system requirements allowing even more of you to try it out. (macgamerhq.com)
A new mod system allows you to tailor your character to your particular style, and the varied environments provide an incentive to keep digging and new challenges to contend with. (video-games-galore.com)
What's more, when you start developing a taste for racing outside of your comfort zone, you can start playing with the brand new Mod system to earn rewards. (gamestm.co.uk)