A lot of Simmers live by this mod, and I can see why: the vanilla game can feel a bit static in the fact that the other Sims in your neighborhood don't do anything without you; now with MC Command Center, life goes on, whether or not you play each individual household.
Before we get down to the nitty gritty, you should probably learn how to install a Mod into your game.
The Sims 4 makes it easier than ever to install mods: simply download the mod in question, unzip the files (they will typically end in .package), and then drag and drop them into the "Mods" folder, which is already located in your Sims files (navigate to Documents *This hub will be regularly updated to include the newest/coolest mods! Fans have been creating Mods and sharing them online since the Sims 2 was released.
MC Command Center is what we used to know as "story progression", which wasn't included in The Sims 4.
This mod allows your neighborhood to grow and thrive without your direct interactions.