Onion skin mode displays content drawn on the current frame plus content drawn on the surrounding frames. These additional strokes appear at the opacity you specify to distinguish them from the strokes on the current frame. Onion skin mode is useful for drawing frame-by-frame animations because it gives you reference points for the stroke positions.
Use onion skin settings to specify how previous and forward strokes (surrounding the current frame) appear when Onion Skin is enabled during playback in the Animation palette.