It allows server admins (or single players) to create custom stat-based achievements for their players to achieve. Creating new achievements is as simple as editing a json file and restarting the server. See the wiki page for more info on it.
This mod is NOT required on the client, so it is an optional mod. Players without it will not suffer from any bugs because of it. But if they do have it installed, they will receive a popup when they get an achievement and also be able to check their progress in the achievement page.
Requires Minecraft Forge and Minecraft 1.7.10. A 1.8 version is also available.
The 1.8 version is recommended because 1.8 has significantly more stats than 1.7.10 for achievement creators to take advantage of. The actual code in either versions hasn't changed.
Source and Copyright
Achievement Get uses the Apache v2.0 licence. You may use it in modpacks as long as you give credit. The source is available on Github.
I seem to be mistaken. The version on curse right now does not have meta support because I never actually released that change. It's been a while since I worked on it, so things are a little gray about what's in what version. Sorry.