Fading is a change in colour with time. All colours fade over time to varying degrees.
The technical term for colour fading is 'photodegradation'.
The UV part of sunlight has a chemical reaction with the pigments in ink, causing the colours to fade in a process called bleaching.
Fluorescent colours include unique pigments that 'fluoresce' - they convert parts of the invisible UV light into visible light, causing fluorescent colours to look so bright.
These fluorescent particles are highly susceptible to bleaching when exposed to UV-light; this is called photobleaching.
Getting more technical; some science behind fading
Dye's in ink contain light-absorbing colour bodies called chromophores. The colours we see are based on these chemical bonds and the amount of light that is absorbed in a particular wavelength.
Ultraviolet rays, visible light and solar heat all break down these chemical bonds and fade the colour.