With a double-click on Constant Line or with a drag and drop on the View, you can automatically add a Constant Line to your visualization. When you add it, Tableau opens a small textbox where you can enter the value of the constant. A Constant Line is an excellent way of representing a goal. 

For example, the goal could be to have sales above $100,000 for each Sub-Category. As you can see in the following screenshot, it's straightforward to spot the sub-categories that are above or below the goal with a Constant Line:

With a simple click on the line, you can change the value of the constant, change its format, or remove it. Let's continue with the second, very similar option; Average Line.