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, ...
