When editing Moodle courses, perhaps by referring to the Moodle Support Hub along the way, you will regularly be asked to Turn editing on.
You will only be able to click on this link, which appears in the Administration block illustrated below when you are an editing Teacher within a given course.
This will also appear towards the top right hand side of the page in the form of a smaller icon
Please be aware that leaving editing turned on during normal Moodle navigation can lead to serious untended consequences. You should therefore only have this functionality enabled when you wish to edit your Moodle course.