Memcached is a general-purpose memory cache server daemon. It can improve website performance by moving Drupal's standard caches out of the database and by caching the results of other expensive database operations. Acquia Cloud supports Memcached for all paid subscriptions. If you are an Acquia Cloud Professional customer, you can enable and manage Memcached for your websites. If you are an Acquia Cloud Enterprise customer, Acquia will configure your servers with an appropriate amount of memory for Memcached.
The most common use of Memcached with Drupal is storing Drupal's cache tables in Memcached instead of in the website database. This reduces the load on the database with every page load.
Using Memcached may not make any noticeable difference to a website that is already performing well at every layer in the server stack. Memcached is most helpful to websites that need to reduce the load on their database servers.