Chart background is a canvas chart is drawn on, which usually outlines the chart itself. "Chart background" dialog windows allows modifying background color and padding between chart and the edges of its display area.
To open "Chart background" window click "Options & Data" menu and choose "Chart background" item or click "Options & Data" tab at Quick access panel and press "Chart background" button.
"Color" control displays currently selected background color and opacity values and allows changing them using color selection dialog.
"Insets" pane has 4 checkboxes (Left, Right, Top and Bottom) that defines the distance between chart and the edges of chart display area (measured in pixels) and insets preview window.