I am referring specifically to the green plus sign in the screenshot below.
[edit] This is taken from "Outline" view, and I am coding in Java.

This might help: What do the icons in Eclipse mean?
From the icon index, the plus means add, the C means public class, and the warning means that there is a java element warning.
So from what I can assume, it means you can add a new public class, but is warning you of possible problems with your current project.
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