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
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. But you need to come up with something in return
for decryption, a kind of some kind of perversion, so that you are
afraid to pronounce. (original comment, translated using Google Translate)