The NDVI (Normalized Difference Vegetation Index) is an indicator of the vegetation vigor. It is suitable for estimating vigor during the initial stages of the crop. It allows to know at a glance the current state of the crop, which can then be compared with another temporal image in order to observe its evolution over time.
This index uses the near infrared and red bands of the electromagnetic spectrum to estimate a dimensionless indicator between -1 and 1, where values close to 1 indicate vigorous vegetation and close to 0, absence of it.
As for how the NDVI is calculated, the standard NDVI formula is used.
NDVI = (NIR - red) / (NIR + red)
The scale to the right of the field is the reference to know the vigor of the crop.