ENBSeries is a graphical modification for many games, including Fallout 4. It uses a modified d3d11.dll file, implementing new graphical options such as ambient occlusion, indirect lighting, Bokeh depth-of-field, lens effects, better bloom, HDR, tone maping, parallax maping, vignette, sun rays, detailed shadows, film grain, reflections and much more to come in later versions.
The settings can be modified and adjusted to your liking. Many people have released their own ENB presets which they use and wish to share.
And no, the creator won't tell us what the ENB name actually means. I'm serious, he won't. Don't attempt to figure out what it stands for and just remember what it does; it makes your games look pretty. Go to the official website for more information.
IT'S TOO BRIGHT
How do I use ENB?
First, you want to get the d3d9.dll file from the official website. Usually the newest release is the best option, but some presets require a specific version because of stabilization or specific issues. After downloading the enb, place the files into the Skyrim main folder (usuallly Steam\steamapps\common\skyrim\). Now if you want to use a preset made by someone else, download and extract it to the same place as before. This should overwrite some files but it shouldn't interfere with the d3d9.dll. Once you start the game, you should see a significant difference between the ENB and vanilla.