When user open a photo either within gallery tab or from a stram, the photo is displayed with the side panel ( info, comments if any...) . The icon >> act as a toggle to show or hide the panel.
Issue which is source of lot of confusion : It seems that there is no 'reset' of this toggle when the photo is closed, then next photo opened may appear with the panel hidden. Users are lost because they do not find the info and comment area.