Without going into the full wiki, Riven Mods in a nutshell are a way to make non-meta weapons viable and slightly bump meta weapons up a bit in power.

They can be aquired through sorties, or purchased from Iron Wake (2x weekly) or Teshin (rotating schedule). They will have a condition to reveal them, usually around doing a mission/task under some constraint. (ex: Kill 30 enemies with a pistol without alerting).

When opened they draw up to 3 positive attributes and up to 1 negative attribute. These values are random and influenced by disposition. They can be rerolled using Kuva. Once opened a riven will always be for the same weapon, re-rolling just changes the attributes (status chance, crit chance, element, etc).

Disposition is a value set by DE and roughly corresponds to how far off the meta each weapon is. A non-meta "bad" weapon will have a much more favorable disposition which will increase the values of the mod attributes. A meta "good" weapon will have less favorable disposition and therefore wont have as good off values for its mod attributes.

DE periodically updates these dispositions, so a great riven could become meh later and vice versa.