Resume Rendering lets you continue a rendering which has been completed - either because the number of passes or rendering time limit has been exceeded, or because you clicked the Stop Rendering button - and add more passes.
IRender nXt is a multi-pass rendering system. The early passes allow you to quickly check settings - such as lighting and materials. With Resume Rendering you can render for just 20 passes or os, see how it looks, and if you like it add more passes, or a time limit, and continue without having to start over.
After rendering, you can either make changes and restart the rendering, or use Resume Rendering to add more passes for better results.