The Element ID is used to target a particular component when adding custom CSS to your Visualization. Each Element and Widget within the Screen Displays and Web Pages Builder has its own ID, so make sure to use the right one when applying your customization.

You can find the Element ID within the Advanced Styling section.

To get the ID, select the Element by clicking on it and you will see the ID on the left sidebar: 

To get a Widget's element ID, select the Widget by clicking on it, then go to its settings, and finally scroll all the way down, as shown in this image: 

Check out how to use the element ID: