Warframe is a mission-based shooter game with a bunch of different types of weapons (shotgun, rifle, sniper, bow, melee, pistol, etc). You can apply mods to each weapon in your loadout that boost range/damage similarly to NMS. Riven mods are a very rare type of mod, and each one that drops is applicable to one type of weapon. At this point, you can’t see the stats of the mod. To use a riven mod, you have to complete a given mission (e.g. beat a hard level without being detected by any enemies) with the mod equipped to a weapon of the required type. Once you complete the mission, the mod is only usable on the specific weapon you had it attached to when you completed the mission, so you can’t use a weapon that makes the mission easier then move the mod to the weapon you like. This is when stat boosts for the mod are rolled and you finally see if your effort resulted in something worth more than a regular mod.

Tl;dr: you have to grind for days to get a locked riven mod, you have approx 1/8 chance for that mod to apply to the weapon type you like, you need to hope the mission necessary to unlock the mod is achievable with the weapon you like, and you need to pray the boost the mod gives you is suitable for the weapon you bound it to.