Well, it is a decade-and-a-half old and fairly common in the Python web world. I don't think there's any shame in not knowing what it is if you haven't encountered it, but I also don't think there's any shame in assuming a developer audience reading about web development would at least know how to figure out what it is.

When I look at the page, there’s a link on the word Jinja. And the next word is templates. Maybe you aren’t familiar with what a template is, but they are hardly obscure. Here’s a list of large websites that used a Perl-based templating engine a decade ago: http://www.masonhq.com/sites

There is literally a link on the word Jinja to the Jinja docs...