Lumens and Candela can help you understand the three main characteristics of a light pattern and distance of illumination.
The three characteristics of light patterns from weapon lights are throw, flood, and spill. Throw is the light's ability to reach out to far distances, like when you're trying to see what's across a field or in a tree line.
Flood is the light's ability to illuminate a wide area immediately in front of you. Spill is the effect produced in a light with a combination beam, where ambient light surrounds the intense spotlight center giving you "best of both worlds" performance.
To understand how Lumens and Candela can inform your understanding of how a light will perform, let's give an example.
If two weapons lights have a 500 Lumen output, but one light has a higher candela, its concentrated beam in the center of the light will be more intense, and constitute a larger percentage of the light's overall lumen output.
A 500 Lumen light with a higher Candela rating will send more of it's power out into the distance, allowing you to see further. The 500 Lumen light with a lower Candela measurement will cast more of its lumen output into the spill light, making the center spotlight less powerful but offering more ambient light immediately in front of you.
Often times, if lumens are equal, the light with a higher candela rating will appear to be brighter, because the center is so much more intense.
This is especially true if you were indoors and shining each light into a wall from a short distance, as the intense center from the higher candela light will reflect more. Each of these values, as well as what they mean to you, is important to consider when selecting the best weapon light for your AR-15, pistol, or specific application.