HDRI Stands for “High Dynamic Range Image”. Which basically means that images can have darker and brighter spots than images with a normal Dynamic Range. Which makes them more detailed and Life like than normal Images. Especially for Environment Textures, this can make a big difference.
Usually those files end with .hdr or .exr.
Where to Download Environment Textures
The Website I use the most to Download Environment Textures is Poly haven. The website has a big range of free and high quality Environment Textures. My favorite Environment Texture is Small Hangar 01. I think it can be really helpful for shading. Also, feel free to check out the Article Best Resources for 3D Artists, in which you can find alternative sites.
Not matter if you are using Cycles or Eevee just switch to the World Properties Tab and click on the Dot on the left side of Color. Then, just select Environment Texture (or press “E” on the Keyboard) and Click on Open to Open your Image file.
Modify an Environment Texture
If you switch to the Shade Editor and over there switch from Object to World you can modify the Image as well as the orientation of the image inside Blender like with a Normal texture.
Common Modification for Enviroment Textures
With a Hue Saturation Value Node between the Image ad the Background Node, you can change those values.
Change Brightness and Contrast
With the Bright/Contrast Node between The Image and Background Node you can change the brightness and contrast.
Supplementing Environment Textures
The Launch spot for Onlyfy one was the last project, in which I used an HDRI as my base for the lightning and supplemented it in this case with a simple Plane with a blue diffuse Material.
This makes the bottom of the laptop reflect the blue color, which helps, to make the object seem more integrated in the Background.
There are all kind f things you can do, like adding an extra light to emphasize something. Or create a shadow to make something less visible.