Items (tasks, features, links, and nodes) can have one of several statuses. The explorer shows item statuses by using status icons in front of the task type and feature icons.

When working on a shared model, you will have downloaded a work set. In the Explorer Pane, you will find status flags in front of each item, informing you about what items you may work on and what items you have edited but not yet shared, and so on.

This image sums up the different possible statuses.