In case that’s Greek to you, APC stands for Alternative PHP Cache and is a PHP extension that provides an opcode cache for PHP: it caches data and compiled code in memory. From the readme:
This plugin is designed to remove a lot of the database traffic from YOURLS, primarily the write load from doing the logging and click tracking. We have attempted to strike a balance between keeping most information, but spilling it in some cases in the name of higher performance.
If you have a server capable of running APC, especially if you’re running a popular URL shortener, please give the plugin a try and send Ian some feedback!