Configuring user agents is optional.
Every browser has a different user agent and this allows you to uniquely identify particular browsers your customers may be using. Evolved Caching allows you to cache pages separately according to user agent. This is great if you have for instance a mobile store running under the same domain as your main store and want to cache this separately (Magento allows you to configure a mobile theme in this way by user agent), or perhaps you render some content out differently for different browser to cater for older browsers with less support.
If you want to configure user agents, you can do this in the User Agent section. You should enter user agents as a comma separated list, and you can group user agents to cache together by using standard brackets. So if you might enter:
Here, agent1, agent2 and agent3 would all be cached together, agent4 would be cached separately, as would agent5. From version 1.7.1 of Evolved Caching matching against user agents entered here is case sensitive.