The sidebar is the area to the right of your spreadsheet that shows either the Format inspector or the Organise controls.
Format: Click in the toolbar to show formatting controls for whatever is selected in the spreadsheet, like a table, chart, text or a shape. Click the tabs at the top of the sidebar to see more controls.
Organise: Click in the toolbar to show controls for creating sorting rule, filtering and categories for the spreadsheet.
If the sidebar on the right is open and you click the button in the toolbar for the active controls, the sidebar closes. For example, if the Format button is selected (with the Format controls in the sidebar) and you click , the entire sidebar closes. Clicking keeps the sidebar open and switches to the Organise controls.