Lock is a great tool, especially when you have inserted a graphic element which you do not want to accidentally move or edit while making changes to other elements on the same canvas.
To do so, simply select the element (or group of elements) you would like to lock. Then, click on the padlock icon found at the top menu.
You know you have locked an item when the handlers and boundary lines around a graphic element are red in color.
You may also have encountered locked items when using Pikto-themes. The red boundary lines and handlers simply mean that these items are locked. To unlock, simply click on the lock icon on the menu.