It is a chart showing your profit and expenses for the specified period year. Amounts are expressed in the base currency. You can select one of the following chart types in this widget: bar chart, where results are grouped by months and average income line is painted, or linear view, showing income in sequential order (where current month income is highlighted with red color).
To find out how values in this chart were calculated, you can always open Reports - Ledger, and check records per necessary month. Please remember that earnings can be calculated differently depending on the accounting type you use in the application. Detailed information about earning accounting type can be found in this article.