This mod enables Sims to perform most interactions on sloped terrains.
When Sims socialize with other Sims or interact alone (like playing guitar), the game will try to create a "jig". A jig is an invisible pad where Sims have to stand in order to animate correctly, e.g. a Sim will not be too far or too close to the target Sim when they're flirting, etc. This is why Sims always try to "position" themselves whenever they're ordered to socialize. The problem with this is that like most objects, a jig cannot be placed on a sloped terrain, which causes Sims unable to interact.