Light you can't see
A normal photo only uses the sliver of light your eyes register - red, green, blue. Sentinel-2 measures well beyond that, and two of those invisible bands are exactly the ones that reveal plant condition earliest.
Drag to compare - true colour on the left, near-infrared on the right.
Vigour
A healthy leaf's internal cell structure reflects a lot of near-infrared light - far more than it reflects visible red. As a leaf thins, yellows, or dies back, that reflectance drops well before the change is visible to a person walking the plot. Comparing NIR against red gives NDVI, the standard vegetation-vigour index.
Moisture
Water absorbs short-wave infrared strongly, so leaves and soil holding less water reflect more of it. Comparing NIR against SWIR gives NDWI and NDMI - two moisture proxies for canopy and soil that dry out before the plot looks dry.