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It helps if you separate the first three letters:

JavaScript + APIs + Markup = JAMstack.

Markup is the content, which can be entered via any method. You can set up a CMS like WordPress or Ghost to serve this role, or you can build your own interface. That markup is then made compatible with various JavaScript libraries via APIs.

But it evolved over the years to the point where "JavaScript" part isn't entirely accurate. You can throw any static site generator in the mix and just use a plugin that will convert content edited via CMS into Markdown files.

It's basically a way for front-end developers to not particularly care about the backend. So instead of making WordPress themes or whatever they make a site using static site generators. Users don't like the look of an editor? You have the ability to change it completely, find a plugin, and the end result is the same website. No need to adapt the theme to the new CMS or bother with migrating content between editors.