It's a set of libraries designed for creation of server side content, that work for player's without mods or (required) resource packs! You can create blocks, items and entities, that not only will work fully on server side (and singleplayer), but also are represented on server the same as normal (vanilla/modded) ones allowing for way better mod compatibility and way less weird edge-cases.
It is also should be fully compatible with close to all other mods and in case of found issues they are patched as soon as possible.
Updating from 0.2.x
With Minecraft 1.19.3 being globally breaking update. I took this as a good time to make breaking changes to polymer itself.
It's a heart of Polymer. It allows you to create server side content. It also contains lots of extra mod compatibility for client mods, to make your mod better fit for any modpack.
Polymer Resource Pack
Allows creating global (and mod specific) resource packs. It also patches PolyMc to make it's resource generation work with polymer.
Polymer's Networking API. Uses its own custom synchronization code. Contains extra API, more specific that aren't available in fabric.
Extension of Polymer Core and Resource Pack. Allows creation of textured blocks.