I am surprised that almost 1/3 of voters don't know what JIP LN is.
Well I will leave here a quick description of it.
JIP LN is a merge of two NVSE (New Vegas Script Extender) plugins, JIP's (made by jazzisparis) and Lutana (made by ). These plugins add hundreds of new functions to NVSE allowing modders to do things that are just impossible by using the vanilla game or the game +NVSE.
These functions also allow better stability and performance for things that could only be possible in the past by using workarounds.
Many mods require this plugin anyway (for example, if you go on nexus and search on the mods description "Lutana", "JIP LN" and/or "JIP NVSE" and you will get several pages of mods). It is pretty much an essential plugin for modders that use NVSE and for users who use those mods (obviously).
It is just a tiny .dll file that sits in your NVSE plugins folder, but allows a mindblowing number of new and powerful modding capabilities.