Normalized Difference Vegetation Index (NDVI)
The NDVI for its acronym, quantifies the vegetation by measuring the difference between the near infrared (which strongly reflects the vegetation) and the red light (which absorbs the vegetation).
The Normalized Difference Vegetation Index (NDVI) uses the NIR and red channels in its formula
NDVI = (NIR - RED) / (NIR + RED)
The result of this generates a value between -1 and +1. If you have low reflectance (or low values) in the red channel and high reflectance in the NIR channel, this will produce a high NDVI value and vice versa.
Healthy vegetation (chlorophyll) reflects more near infrared (NIR) and green light compared to other wavelengths. But it absorbs more red and blue light.
That is why our eyes see vegetation as the color green. If you could see near infrared, it would be strong on vegetation too.