Memcached Stats is a plugin that provides a admin dashboard widget that displays stats for each of the memcached servers in your MT config.
Memcached is an optional advanced feature with MT4. You can read more about memcached here, and about using MT with memcached here. In short, memcached is a caching system that caches data in memory. When configured for MT4, Movable Type will store numerous items in the cache to reduce the number of database queries and improve performance. To use memcached, your probably need a dedicated server with root access, as you have to compile and install several libraries. I installed it earlier today with help of this tutorial.
Movable Type versions: 4.3