In photo printing it is a term used to describe the durability of printed digital photos
and how well they stand up to exposure to light. Sunlight and other artificial lighting can cause a reaction on ink pigments and can cause the color to fade or change over time with exposure. In printing, when an ink is called lightfast, it means the manufacturer states it has a strong light-fade resistance.
Compare with gasfastness, the durability of a print when exposed to elements other than light.