ENBs are mods the make your game look absolutally outstanding, they add new effects and change the feel of your game visually. many games utilise this including the witcher 3, The Elder scrolls 3: morrowind, The elder scrolls IV: Oblivion, The elder scrolls V: SKYRIM and of course fallout 3 nv and 4.
downloading and installing an enb can be a tidious proccess so here is a write tutorial:
1: download the enb binaries (i am not allowed to post the binaries here, and the binaries are incredibly important, its like installing a skyrim mod with out skyrim) http://enbdev.com/download_mod_fallout4.htm
2: drag and drop the contents to your games install folder (steam/steamapps/common/{insert game name here})
3: download an ENB preset from Void mods or anyother modding site such as nexus
and follow the same proccess as step 2 and click yes to any pop ups saying "overwrite"
the enb is installed but if you want to change somthing like shadows or lighting press shift+enter to bring up the enb menu (diffrent enbs might have diffrent hotkeys)
make sure you click save if your modifying an enb
here are some of the existing ENBs avaliable right now