One of the elements of Fallout 4 that was most touted prior to release is also the element that is probably the most heavily modded: F4 settlements. Although changes and additions were made with the game's official add-ons, there's still plenty about settlement building and settler duty-assigning that's pretty rough around the edges.
Sim Settlements is much different from the typical settlement mod in that it drastically changes how settlements work, rather than just simply adding new objects.
The goal of the settlements mod is to make it so you are the leader of any given settlement -- and not its micro-manager. The mod brings in "zones" that can be placed (in the manner of Sim City) that prompt your settlers to plant crops, connect water supplies, and construct buildings of a specific type.
Every settlement will end up unique using this overhaul, and a progression system is added in so that building settlements is more satisfying and less of something you have to do every so often.