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.