The map legend conveys the meaning of symbols on a map. It is particularly useful when the map has multiple operational or thematic layers. The legend element displays the legend of a map element that is added to your dashboard. 
Since the contents of your web map determine the contents of the legend element, the order in which layers appear in the web map reflects the order in which they appear in the legend element. If a layer's visibility is hidden in the web map legend, it is hidden in the legend element as well. Additionally, the legend element respects visible scale ranges in your web map. For example, if a layer is not visible in your web map due to scale range thresholds, it does not appear in the legend element.
Note:
The map legend element requires a map element to be added to your dashboard first. If you have multiple map elements, you need to specify which map the map legend is based on.
The map legend element is different from the legend map tool. The map tool is part of the map element and is only visible when clicked. The map legend element is always visible after it's added to a dashboard, unless it's stacked with another element.