The Plone logo has several connotations:

Find what you're looking for

The original draft of the logo singled out the three dots within the circle from other dots. The outside dots were later removed, since it makes for a cleaner logo.

Collaboration

The logo has three dots in a group, representing collaboration.

Pluggable, extensible, amplifies your voice

The Plone logo looks like a plug, signifying that it works well with other systems, that they can be integrated and talk to Plone, and that Plone can be extended easily. The fact that it looks like an XLR plug — a common connector for microphones — has also been noted.

Open

The Plone logo is the Braille (a system used by blind people to read and write) letter "O" for Open. Plone has a special focus on accessibility for the blind and motor impaired, as well as translations for all major languages (over 35 different translations exist as of 2006) — aiming to make Plone accessible to people of all languages, and regardless of motor or sight impairment.

There's also a few sillier explanations: Petri dish (representing the organic community and culture), the right angle bracket ">" (which is important in HTML, which is the language Plone's web pages are using), and others.

Of course — as with all good logos — the explanation/rationale/associations always come after the logo has been created. What associations did you get when you first saw the logo?