Light meters are devices used to measure the intensity from a light source given the distance in which the meter is held from that source. They capture the photons emitted from the light at a particular point on a plane.
For example, if you were to hold a light meter 4 feet from your light, you would be reading the PPFD or PBFD (depending on the type of light meter you are using) at one particular point, 4 feet from that light. The reading from the light meter does not tell you the full picture of how much light is coming from the source. It is a singular data point. Only when you put together multiple data points, will you have a full picture of the intensity and uniformity from your light. The more points, the better.
There are many types of light meters and new ones are being developed all the time. It is important to find a light meter from a reputable company that is inclusive of the entire spectrum.