A colon is a diacritical mark that looks like this:
(that's a joke, folks ;)
It works a bit of technical magic in the Mediawiki software that is the foundation of our wiki.
A single colon at the beginning of a line will indent that line. Use additional colons to indent further. This can also be used in combination with * (which creates a bullet) or # (which creates a numbered line) in fancy ways.
Files, images, and categories
Files, images, and categories all have wiki pages associated with them. However, if I put in the standard wiki code to make a link to an image, it instead puts the image right on the page! For instance, the code [[File:UniversalEditButton.png]] will just put the image on the page:
But what if I instead want to LINK to the image? To do that, just include a colon after the opening double brackets: [[:File:UniversalEditButton.png]]
And you'll get: File:UniversalEditButton.png
The same thing applies to categories.