Annotations can be used to indicate significant changes to your website that might effect your rankings in the Google search results. To add annotations to the "Ranking history" graph, you can either use the "Add annotation"-text below the graph:
Or you could go to the "Annotations"-tab and press the "+ Add annotation"-text
When either of the "Add annotation"-texts have been pressed, a window will show, asking you for the date of the annotation and the text within the annotation. Adjust it after your needs and press "Add".
All added annotations are available under the "Annotations"-tab and depending on when they were made they will also be visible on the graph in the Dashboard.