Soft Proofing
Simulating on a calibrated screen how an image will look when printed on a specific paper and printer.
In depth
Soft proofing applies the output profile to the screen preview, showing the gamut compression and the paper's reduced contrast before any ink is used. The 'simulate paper colour' option is the important one, since it drops the screen's brilliant white to the paper's actual tone — usually a shock, and the reason prints look flat when this step is skipped.
Examples in practice
Soft proofing reveals that a saturated sunset will lose its deepest oranges on matte rag, prompting a targeted adjustment.