Making asynchronous requests to fetch data. In this context specifically, the common case of fetching data asynchronously and then rendering it to the DOM.
My impression is that Jamstack is meant to include apps that render client side, but disclude apps that require a custom dynamic backend - serving static data and fetching it asynchronously is in, as is hitting third party APIs (the API part of JAM), but writing your own backend to serve data from a DB / etc. is out. Does that make sense?