The primary function of the Mirror feature is to facilitate the virus signature update process while decreasing Internet traffic over an organization's local area network (LAN).

In order to continually provide the maximum level of protection against the latest threats, all ESET client workstations must check regularly for new program component and virus signature updates. By default, ESET client workstations check for updates on an hourly basis. The client workstations can obtain these updates over the Internet directly from ESET’s update servers, or from a locally managed update source known as a 'Mirror server” (so-named because it stores an identical copy, or “mirror” of the update files that reside on ESET’s update servers).

The Mirror server provides you with the ability to maintain a local copy of virus signature and program component updates on your own network. It helps save bandwidth by centralizing the download and distribution of updates to ESET clients, and also allows you to provide updates to ESET clients which are not directly connected to the Internet.

Related:

How do I install ESET Remote Administrator and configure a Mirror server? (3.0)

How do I configure ESET clients to access the Mirror server? (3.0)

How do I create a dual update profile configuration with ESET Remote Administrator? (3.0)