Light intensity is the amount of light measured over a certain area. In essence, it is how much light is being provided.
There are many ways to measure intensity. The type of measurement used depends heavily on the application. Light intensity is traditionally measured in lumens. Lumens represent how intense or bright a light source is to the human eye. You will see this used for places such as our homes, a parking lot, a walkway or the street. Lumens should not be used to measure light in the horticulture industry.
For plant growth, measurements such as Photosynthetic Photon Flux (PPF), Photosynthetic Photon Flux Density (PPFD), Photobiological Photon Flux (PBF) and Photobiological Photon Flux Density (PBFD) can be used to help describe how much light a plant sees. Unfortunately, these numbers can be manipulated depending on certain factors. It is important to be well informed on the metrics you are using and how they were generated when evaluating the intensity of a light source for your plants.
Additionally, the intensity of a grow light has an impact on the overall yield of your crop. Theoretically, the more intense the light, the more a plant will yield. An extremely intense light can also have an adverse effect on yield if too much light is delivered to the plant canopy or the spectrum being delivered does not coincide with the stage of growth.