The quick answer - a static site generator. You create templates using Liquid template engine, and content using HTML and Mark Down and the combo comes together and generates a standard HTML website. The site is generated on your local machine and can be deployed by simply uploading the rendered site or you can utilize many other technologies to deploy the Jekyll site. Jekyll is written in Ruby, but the only code used to build sites is Yaml, Liquid Templates, and Mark down. Jekyll is to HTML and CSS as Less and Sass are to CSS. You can easily write HTML and CSS but Jekyll allows you to do so much more.
Static site generator offer a bunch of benefits. Jekyll sites are super fast, secure, and easy to maintain. When you are coding the site you get good site organization, flexible powerful templating, and the output is a secure, databaseless site that is easy to deploy and render. You don't have to worry about differing server environments, slow render times, or securing a database - you just upload your flat files and go.
Do you need dynamic content? That is a really good question. To answer that you need to ask what your site is going to be used for and how it is going to be used. The first and most important question is if users of your site are going to be editing, or changing content or your site. If users are logging in, updating, changing content, contributing or anything else then it might make sense to avoid a static site generator like Jekyll. If you aren't then Jekyll or a another site generator might be a good choice. Do you need speed? Jekyll makes a static site. That means the files are made and a server only has to serve it. That means you don't need to think about things like a cache or render times or dynamic server languages or anything else. All you need to worry about is uploading it to a server. Do you want something Cheap? Since Jekyll sites are just plain ole HTML you can host it on pretty much anything. Github pages even offer hosting for Jekyll sites. And since they are static, it's doubtful you'd need a super computer to serve the site.
It's Hard to Learn There is definitely a learning curve. It takes a bit to jump into it and get it working. But once you get it running it should be pretty easy to maintain. However Wordpress is complicated in its own right. So this might be a wash if you are just getting started - if you are already a Wordpress user then it might be wiser to stay there. It's not Plug and play Plugins and themes are super easy to utilize inside of Wordpress. That means a novice can easily customize their site making it their own. Moving to Jekyll means their are less choice readily available. Plugins are usually Ruby gem extensions that will have to be installed and all of it is foreign if you are normally used to Wordpress and its ease. It's not Prolific . . . yet Wordpress has been around forever. There are so many resources out there, and if you need help you can find a billion articles. Jekyll is relatively new, and while there are plenty of resources it isn't even in the same ballpark as Wordpress