Jekyll is a web-site generator that filters a site's files through a templating system. It will process any file that has yaml front-matter, so if you want the export of any file to be modified by Jekyll you only have to add something like this at the very beginning of the file:

#+begin_html --- layout: default title: Who knows --- #+end_html

But Jekyll has a special deal for files in the _posts directory whose name is of the form year-month-day-something-or-other.html: it considers them blog posts and makes their exported versions available to the templating system, so you can use them to build RSS feeds and indexes.

Other approaches rely on writing your blog posts in separate files and exporting them to _posts with the right name, which is perfectly good if you want each post it a file. Org-jekyll is for you if you want to litter your outlines with posts, as I do, without having to think too much about where to store them.

See also the tutorial on jekyll and org-mode integration.