function makeSidePanel() {
  // Create the base side panel, into which other widgets will be added.
  var mainPanel = ui.Panel({
    layout: ui.Panel.Layout.flow('vertical', true),
    style: {
      stretch: 'horizontal',
      height: '100%',
      width: '550px',
      backgroundColor: colors.gray,
      border: BORDER_STYLE,
    }
  });

  // Add the app title to the side panel.
  var titleLabel = ui.Label('Collection Mosaic Editor', TITLE_STYLE);
  mainPanel.add(titleLabel);

  // Add the app description to the main panel.
  var descriptionText =
      'This app allows you to interactively explore the effects of ' +
      'mosaicking different Landsat 8 TOA images using ee.Reducer.median(). ' +
      'To change which tiles are included in the mosaick, check or uncheck ' +
      'the thumbnails. To mosaic another area, pan/zoom and click on the map.';
  var descriptionLabel = ui.Label(descriptionText, PARAGRAPH_STYLE);
  mainPanel.add(descriptionLabel);

  return mainPanel;
}
Return the main side panel, populated with title and app description.