In addition to adding text to shapes, you can also label lines with text. To do this, simply select a line and begin typing. A background text object will be created and attached to the line, normally in the center.
This text object will remain attached to line when you move it. It can change its orientation to match that of the line, or it can remain horizontal. You control this using the Align Shapes and Text on the Line command settings, found on the Lines Menu.