It can be used to run a command and embed its output in the built HTML. It can pass stdin or additional command arguments. The repository contains Zola examples.
@keats Your comment about thinking about something similar is from a while ago. I saw on Github that you don’t plan this anymore and I agree this feature shouldn’t be built-in for security reasons. Running arbitrary commands hidden in some template when I try building a static website is not something I would expect or want. But maybe you could add my http-cmd somewhere to the docs. I don’t want it to be communicated as something official. Just in some FAQ as “this is a third-party tool that could be used”.