dispatcher is a "reverse proxy" which is a type of proxy server that retrieves resources on behalf of a client from one or more servers. These resources are then returned to the client, appearing as if they originated from the proxy server itself
from security viewpoint, you can either whitelist or blacklist urls or regex patterns or extensions and thus restrict the traffic flow to allow only what should be allowed from specific users/servers refer - The Dispatcher Security Checklist
flush agent(s) trigger requests to "touch" (modify the timestamp) the .stat file in apache's docroot where the cached content is stored.
load balancing (software load balancing) works through '/statistics' section of dispatcher.any. You may configure that "*.pdf" or "*.jpg" should be fetched from one server while "*.html" should be fetched from other server otherwise you can say all files of projectA from first publish server and projectB from second publish server etc.
# The statistics sections dictates how the load should be balanced among the renders according to the media-type.