Tallneck is a giant creature with a characteristic disk in the place where he should have a head. Thanks to his height he can scan the nearby area. Overriding these machines is vital for you since it enables you to reveal various markers on the world map. You will encounter the first Tallneck soon after you receive the overriding device if you stick to the main story missions.
In order to override the Tallneck you must climb on him first. Sadly, the machine doesn't have any pieces on its legs which you could use to climb. Because of that, Aloy must find a higher platform (for example, a roof of a ruined building) and use it to jump towards the creature. Reaching the disk from there won't be hard since the woman can hang on the bars sticking from its neck. At the top you must simply press the triangle in order to override the Tallneck and scan the area.