The VRayLightRect, also known as the V-Ray Rectangle light or an "area light" is a planar light source. The light's shape can be set as a rectangle or a circular disc.

Enabled – Turns the VRayLightRect on and off.

Color – Controls the color of the light. When using photometric units, this color is normalized so that only the color hue is used and the light intensity is determined by the Intensity.

Intensity – Multiplier for the light color; this is also the light intensity in the units set by the Units parameter.

Units – Choose the light units. Using correct units is essential when using the VRayPhysicalCamera. The light automatically takes the scene's unit scale into consideration to produce the correct result for the scale you are working with. The possible values are:

Default – The color and multiplier directly determine the visible color of the light without any conversion. The light surface appears with the given color in the final image when seen directly by the camera (assuming there is no color mapping involved).
Lumens – Total emitted visible light power measured in lumens. When this setting is used, the intensity of the light does not depend on its size. A typical 100W electric bulb emits about 1500 lumens of light.
lm/m/m/sr – Visible light surface power measured in lumens per square meter per steradian. When this setting is used, the intensity of the light depends on its size.
Watts – Total emitted visible light power measured in watts. With this setting, the intensity of the light does not depend on its size. Keep in mind that this is not the same as the electric power consumed by a light bulb for example. A typical 100W light bulb only emits between 2 and 3 watts as visible light.
w/m/m/sr – Visible light surface power measured in watts per square meter per steradian. When this setting is used, the intensity of the light depends on its size. 

Is Disc – When enabled, the rectangle light has the shape of a planar disc.

U/V Size – Determines the width/height of the light measured in scene units. The width/height becomes twice this value.

Directional – When this value is 0, the light from the LightRectangle is spread out equally in all directions. Increasing this value makes the light beam more narrow and concentrates it in one direction.

Use Texture – Enables the light to use a texture for the light surface. It is best to have GI enabled if there are surfaces that are close to a texture-mapped light. This allows V-Ray to use combined direct and indirect sampling for the light, reducing the noise for surfaces close to the light.

Rect Tex – Specifies the texture map when Use Texture is enabled.

Texture Resolution – Specifies the texture resolution.

Invisible – Controls whether the shape of the light source is visible in the render result. When this option is enabled, the light source is not visible in the rendering. When this option is disabled, if the light source is seen directly by the camera or through refractions, it appears in renderings in the current light color.

No Decay – Normally the light intensity is inversely proportional to the square of the distance from the light (surfaces that are farther from the light source receive less light than surfaces which are closer to the light). When this option is enabled, the light's intensity does not decay with distance.

Double Sided – When enabled, light emits from both sides of the plane.

Affect Diffuse – Determines whether the light affects the diffuse properties of the materials.

Affect Specular – Determines whether the light affects the specular of the materials. This means glossy reflections.

Affect Reflections – Determines whether the light appears in reflections of materials, both perfect and glossy reflections.

Diffuse Contribution – A multiplier for the effect of the light on the diffuse component.

Specular Contribution – A multiplier for the effect of the light on the specular component.

Cut-off Threshold – Specifies a threshold for the light intensity, below which the light is not computed. This can be useful in scenes with many lights, where you want to limit the effect of the lights to some distance around them. Larger values cut away more of the light; lower values make the light range larger. When this value is 0.0, the light is calculated for all surfaces.

Shadows – When enabled (the default), the light casts shadows. Disable this option to turn off shadows for the light.

Shadow Bias – Moves the shadow toward or away from the shadow-casting object (or objects). Higher values move the shadow toward the object(s), while lower values move it away. If this value is too extreme, shadows can "leak" through places they shouldn't or "detach" from an object. Other effects from extreme values include Moire patterns, out-of-place dark areas on surfaces, and shadows not appearing at all in the rendering.

Shadow Color – Controls the color of shadows for this light. Note that anything different from black is not physically correct. 

Caustics Subdivs – Only used when calculating Caustics. Lower values mean more noisy results but render faster. Higher values produce smoother results but take more time.

Caustics Multiplier – This value is a multiplier for the generated caustics by the selected object. Note that this multiplier is cumulative - it does not override the multiplier in the Caustics render settings rollout section.