When labeling or reviewing a sample, you can create a new issue by clicking the icon in the top navigation bar. The issues functionality works as follows:
Both labelers and reviewers can open a new issue on a sample. For example, a labeler could open an issue because he's uncertain about how to label something, and a reviewer because he discovered a mistake in a labeled sample.
When an issue is created, it automatically becomes an "open" issue, as indicated by the red icon.
Reviewers and admins can "resolve" issues by clicking the checkmark icon. A resolved issue is indicated with a green icon.
Everyone can add replies to an issue. This allows for back-and-forth communication between labeler and reviewer. Note that rejected samples always go back to the original labeler, and corrected samples go back to the original reviewer.
Reviewers and admins have access to an issues tab, where they can see the list of all open and resolved issues.