NDVI or Normalized Difference Vegetation Index is a simple graphical indicator that can be used to analyze remote sensing measurements, typically but not necessarily from a space platform, and assess whether the target being observed contains live green vegetation or not.
NDVI maps are being used in agriculture (precision agriculture), forestry, ecology and more. Healthy vegetation (or chlorophyll) reflects more near-infrared (NIR) and green light compared to other wavelengths. It absorbs more red and blue light. This is why our eyes see vegetation as the color green. This is also why more near-infrared is reflected back into satellite or airborne sensors. NDVI maps use NIR and red channels to measure of healthy vegetation.
The NDVI formula is:
Here are some NDVI examples from NOAA: