Up to now, you may have had Plone installed on a staging server to let your content contributors collaborate. Its performance may even seem acceptable if traffic is low. But exposing a simple Plone installation to a larger audience would deliver a slow, insecure experience for everyone.
Plone's rich feature set means that it does a lot of work on each request: checking security on every involved object, walking along the acquisition chain, and rendering TAL templates. As a result, delivering a snappy experience on a popular site would seem to require a large investment in high-end hardware. In addition, Zope doesn't speak HTTPS on its own, so any passwords would be passed unencrypted, vulnerable to anyone in the right place with a packet sniffer.
To solve these problems, a production Plone setup adds a smorgasbord of different server processes, each offering some element of security or scalability to throw into the pot. At the end, a delicious Plone stew emerges, ample...