Riven Slivers are an item in Warframe that can be used to make Riven Mods. They can be gotten by opening Requiem Relics, or as potential rewards from Railjack missions.
10 Riven Slivers can be handed into Palladino, who is located at Iron Wake on Earth. In return, she will give you a Riven Mod, but this can only be done once a week.
Riven Mods are potentially mighty mods for weapons. When you first get them, they need to be unlocked by completing a specific challenge. The challenge may be to perform certain actions or kill certain enemies under different conditions. Once it is opened, it will randomly roll out for a specific weapon. All weapons have a different Riven Disposition, which denotes the likelihood of a mod for that weapon appearing.
Riven Mods can provide several buffs, and potentially a negative, for the weapons they can be used on. They can impact stats like damage, critical chance, critical damage, status chance, range, or rate of fire, to name but a few. It is possible to reroll the Riven Mod and get a new set of buffs by using Kuva. This can be done at the Mods console in your Orbiter.
When you reroll a Riven, you then have the option to keep the initial set of buffs or switch to the new set of buffs, allowing you to pick between the two sets to keep the best one. The Kuva cost of rerolling a Riven Mod starts at 900 Kuva, increasing to a maximum of 3500 Kuva for the 10th reroll, then 3500 Kuva for each reroll after that.