AutoRegLib is a library mod used for various things, including registries and other abstractions. The mod is made by Vazkii, one of the top mod creators. Vazkii and other developers use AutoRegLib to make mods like
Quark (and add-ons)
Akashic Tome,
Battlefields
HeartDrops
Integrated Dungeons and Structures
Magical PSI
Mob Inhibitor
Morph-o-Tool
PickleLib
PotionFingers
Potion Rings
PSI (and add-ons)
and many more. This mod is not needed by itself but is needed if you want to play a mod which requires it.
The main part of the mod is the RegistryHelper, which is used for Abstract Registration by allowing you to register them in a single place. There are also wrappers for creating a BasicItem Object and other stuff that will automatically register upon initialisation of the class.
Outside of registries, there are also many other things AutoRegLib can do, such
as Rendering
Item NBT
Rotation
ToolTips
Vanilla Packet Dispatching
Inventory
Ticks
and much more. It provides lots of easy-to-use utilities and, unlike many library mods, is not only used by the person who wrote it but is also commonly used by third parties.
AutoRegLib is primarily written for MinecraftForge, an Architectury port was in development at one point, and an unofficial fabric port exists.