They store information about lights for the moment the map got

created.

Since your image stays in the background, it will reflect on glossy

surfaces across the scene.

It may work as the background for your scene.

For instance, if you get an HDR image with bright sky daylight and add it

to your scene's background, it will generate the same type of lights for

the render (Figure 6.16).

Figure 6.16 - Daylight HDR

How to use an HDR in your background? Add an HDR in the World

Properties tab at the Properties Editor. There you will find a field called

Surface options. Press the "Use Nodes" button and go to the Color field.

Click at the small button with a yellow circle on the left, and pick

"Environment Texture" (Figure 6.17).