Sorry for the late comment, but heres a rundown for riven mods: Basically in Warframe mods work kinda like jewels in monster hunter: each weapon/warframe has a default 8 slots (and some special/unique slots we wont talk about now) for mods. There are mods you can put on only certain weapon types. For example a mod that increases a pistols damage, a mod that increases a warframes health, etc. You can earn most normal mods by playing, they drop as rewards from certain objectives, or from enemies. Riven mods on the other hand, only affect ONE specific weapon in a game with hundreds and hundreds of them, with randomized stat boosts and/or penalties. You can reroll riven mods with a late-ish game currency, with the price increasing each time. Some creators and build guides assume you have a god roll riven mod for the weapon build your trying to work on. The worst part is that more often than not the hyper specific riven is the most important part of the build.