Rendering a page on the server before sending it. It trades some interactivity for arriving already readable.
A modern initialism naming the choice to build a page's HTML on the server before sending it, coined in contrast to client-side rendering as browsers gained the power to build pages themselves.
The trade that lets a page arrive already readable, at the cost of some interactivity being wired up after the fact. Speed to first sight, bought with a second step.