The premise is that the planet starts about (with default settings) thirty days away from beibg destroyed by a meteor. You and the other couple dozen or hundred people on the server have the obvious goal of stopping that meteor. But nobody actually makes you do it and since you all start with stone tools and wheelbarrows, none of you even have the means to do it in the beginning.
The idea is that you band together with other like-minded players and form a settlement and each of you specializes into a different set of professions (for example, I am a shipwright and logger mainly but also have a small pottery workshop going). In time, you find new ressources or ways to utilise already discovered ressources to eventually build cars, boats, larger settlements and stuff. While that is happening, you can (and probably want to) set some rules for what is allowed and forbidden in your settlements radius (you widen that radius by increasing culture, mostly via decorative items). The rules you set (and players actually have to vote for and come to agreements with) almost always follow a simple “If x then y (else z)” programming logic and can be incredibly creative. Once voted for, those rules are law and can’t be broken by the subset of people affected by that rule. Seriously, one town on my current server basically gutted themselves accidentally by miswording a law. They intended a specific player to be forbidden of doing anything in their town but the wording was "If {name} is resident then prevent ". But since, yes, that player on the server was a resident of something (another town or their own homestead, doesn’t matter), so condition true, every citizen in town was banned from doing anything meaningful, since it wasn’t worded as “prevent {name} from doing xyz”.