This refers to TERA Toolbox (formerly known as "Proxy"), a modular system for the MMORPG TERA used to reduce lag or run script mods.

A acts as an intermediary between your device and Terabox’s servers. When you use a proxy, your internet traffic routes through a different IP address before reaching Terabox.

The proxy server might be offline or overloaded. Check your proxy provider’s status page and change your location port.