File – Save Project saves a 3MF file containing all objects, settings, modifiers and their parameters. It’s essentially a complete snapshot of PrusaSlicer.
When you open a project file on a different computer, it should still produce the same G-code.
We always save project files when we slice something (unless we're 100% sure that it's a one-time thing). If you re-visit the project later on and want to modify something, it's really handy that you can instantly load the exact same settings you used when you first printed it. Plus it's the best way to share the model with someone who also owns a Prusa printer.
Unlike an STL file, which only stores the geometry of an object (as a series of surface triangles), a 3MF file is able to store a variety of different files in a single archive, such as:
Learn more about 3MF here.