'Ground Truthing' is comparing your results to the conditions in the field. This is necessary to do in order to fully understand your FarmLens products, specifically, your NDVI field health map. This is because our 'NDVI' is actually a 'DVI'. This means that he health values shown in the product image are RELATIVE rather than concrete values. In other words, the colors show the range from the least healthy part of the field to the most healthy part of the field. It is up to you to establish what 'least' and 'most' healthy actually entail within that field.
The best way to do this is to open the NDVI map on your mobile device and scout specific points in your field. For example, you may want to scout the least healthy point and then the most healthy point. If you have cellular reception at the field, you can do this in the FarmLens map view. If not, you can download the NDVI map beforehand and use an application like PDF Maps. In both PDF Maps and FarmLens map view, you can see your own location in the field and mark specific points with notes and photos.
For a detailed explanation of how to do this, see this page.