Jekyll is an open source project and as it is explained on the official website:
Jekyll is a simple, extendable, static site generator. You give it text written in your favorite markup language and it churns through layouts to create a static website. Throughout that process you can tweak how you want the site URLs to look, what data gets displayed in the layout, and more.
Initially, Jekyll was - and still is in a lot of ways, targeting DIY and tech savvy people, or even hackers. It is intended to run on your local computer with the command line as an interface.
Stastic mission is to make it possible and enjoyable to use Jekyll online, with the help of Github pages.