Label (or Text) is represented with the   icon.

There are two different cases: when you use the Text Mark type and if you use any other type. 

If you use the Text Mark type, this property is also named Text. You can drag and drop multiple fields on Text. If you put a Measure, the aggregated value is displayed, but if you put a Dimension, all the different values are displayed (multiplying the number of Marks). 

If you click on the Text button, you'll find an option to change the alignment and, if you click on the box with the three dots (...), Tableau opens a new window. This window is a text editor where you can modify the font, the size, the color but, more than that, you can write any text you want. Here's an example of Sales and Quantity in Text with the Edit Label window and the result in the View:

If you use any Mark type other than Text, this property is named Label. You can also drag and drop multiple fields on Label. A Measure is simply displayed in the visualization; a Dimension also splits the number of Marks. When you click on the Label button, you'll find many options:

• Show Mark labels is the same as clicking on the  icon in the toolbar.
• Label Appearance allows you to change the text, the font, and the alignment.

• Marks to Label allows you to choose which Marks will have a Label. You can add a Label to all the Marks, only to the maximum or minimum, only if selected, or to the highlighted ones. 
• The last option allows the Labels to overlap. By default, Tableau chooses to show the Labels only if they don't overlap. 

The following screenshot is an example of Label use:

The next property is elementary but also very useful!