APC Object Cache provides a persistent memory-based backend for the WordPress object cache. APC must be available on your PHP install.
An object cache is a place for WordPress and WordPress extensions to store the results of complex operations. On subsequent loads, this data can be fetched from the cache, which will be must faster than dynamically generating it on every page load.
The APC Object Cache backend is also compatible with Batcache, the powerful full page caching engine that runs on WordPress.com
Be sure to read the installation instructions, as this is not a traditional plugin, and needs to be installed in a specific location.