What is an ENB?

ENB is a series of graphical enhancements for a variety of video games.

It is developed and maintained by Boris Vorontsov. It is an extra set of rules appended to the Direct3D drawing process used by many games, and is strictly post-processing (meaning it adds enhancements to the graphics of games on top of them, so to speak).

It can add, change, or enhance Ambient Occlusion, Depth of Field, bloom, anti-aliasing, colour correction, HDR, and similar shaders and effects.

Additionally, it has the option to improve the drawing of graphics, something known as ENBoost (available for a range of games, like Fallout 4 and Skyrim), "that enhances the efficiency of video memory management, resulting in increased performance and stabilization". ^(source)

There are presets available everywhere (notably on 'the Nexus'), suiting any kind of hardware setup.

As per the official website:

ENBSeries is the project of graphic modifications for games. The main idea is to allow every gamer to configure [what a] game looks like [suiting] their own taste, so every player could share settings with others. Many games may look much better and ENBSeries is the easiest way to improve them.

It work[s] by modifying [the] render functions calls of the games and applying additional effects. ^(source)

The term ENB has an obscure background, but Boris himself stated that:

This is the name and surname of the girl who for a long time was my muse, so to speak. I think I should come up with something else instead, maybe some kind of perversion name, so that people become afraid to say it. ^{(original comment, translated by native Russian speaker)}

People usually come up with funny explanations what the abbreviation stands for, but I will refrain from doing that.

The acronym stands for Enhanced Natural Beauty, a term coined when people started modding the general graphics of Elder Scrolls Games trying to make it look more natural, mainly by manipulating the lighting.

The Nexus Mods Wiki also mentions that it means Enhanced Natural Beauty.

A simple search finds this explanation:

ENB is a graphic mod that enhances post processing effects to make games look better graphically. It’s really hard to describe ENB mods with words, so I will list some of the most popular ENBs here and you can judge yourself by looking at the screenshots provided on the mod’s respective pages. First you will have to get the binary files (source files) for ENB on it’s site [http://www.enbdev.com/download_mod_tesskyrim.htm] Note that some ENBs use different ENB versions respectively. So consult on the mod’s page to see what ENB version it is using. ENBs generally require good graphic cards (GPU) to run properly, so expect performance drops (even if you have a really good GPU, performance drops will still be there no matter what).

Source