Let's take a look at just a single

Let's take a look at just a single clip in the project. It is layered with a number of indicators:

Surrounding the iMovie clip above are the following elements:
 the "Intro" tag is a chapter mark (mainly useful if you're outputting for DVD)
 the "Find better shot?" tag is a comment, the leftmost icon represents a themes transition
 the rightmost icon represents a cross-dissolve transition
 the bottom green bar represents an audio track
 the black sun icon indicates that the clip has had some video adjustments applied to it through the Inspector
 the black speaker icon indicates that the clip has had some audio adjustments applied to it through the Inspector. Muted clips have no "sound waves" emitting from the speaker icon.
 the black crop icon indicates that the clip has had some cropping, rotation, or zoom effects applied to it using the Crop tool