In my original design, the colours are red, blue and green, represent the three primary colours of light, as well as the three colours that were used for links on MediaWiki projects at the time (The current default theme has changed the green links to a lighter shade of blue.)
The white space in the main circular area forms a stylised "w" representing "wiki". This allows the green area to represent a book, which is a symbol of knowledge. The blue arc surrounding the book represents "encompassing" that knowledge, which is something that the Wikimedia projects hope to do. The smaller red circle interrupts the blue arc, showing the incomplete nature of a wiki. Completing the arc would imply that all knowledge has been encompassed, which is a practical impossibility. The combination of all these elements forms a humanoid, with the red circle being the head, the blue arc being arms and the book of knowledge being the heart/torso.
The colours and typeface (Gill Sans) I chose for this design, were adopted as the official colours and typeface for Wikimedia according to the original Wikimedia visual identity guidelines (Archived). Since then, the logo and guidelines have evolved to remove the colour, and change the typeface to the Open Font Montserrat.
Many Wikimedia projects still retain many elements and the colour scheme of my original design:
The design choice for this was to take elements from the design of the foundation logo, and re-purpose them.
The gestalt of the logo is effectively a talking head. The arcs being stylised sound waves as the circular head says something.
The typeface (Lassigue D Mato) was chosen to be a script as a quote is a sometimes a quick quip, or something transitory, something this particular typeface epitomises.
The fading in colour of the arcs mainly is a fading of the intensity of the sound waves and was an aesthetic decision, as opposed to a symbolic one.
If anyone wants more clarification or has any other questions, feel free to email wiki (at) dwev (dot) com.