Since these variables are objects, we cannot assign to them a statement like label_text = "Hello World!". Instead, each variable exposes a get and set method. Let's have a play with these in the interactive shell:

>>> from tkinter import *
 >>> win = Tk()
 >>> sv = StringVar()
 >>> sv
 <tkinter.StringVar object at 0x05F82D50>
 >>> sv.get()
 ''
 >>> sv.set("Hello World!")
 >>> sv.get()
 'Hello World!'
 >>> sv.set(sv.get() + " How's it going?")
 >>> sv.get()
 "Hello World! How's it going?"

These variables are passed to widgets inside their keyword arguments upon creation (or at a later stage, using configure). The keyword arguments expecting these special variables will usually end in var.  In the case of a label, the argument is textvar.