Tkinter is a module in Python that stands for Tk-interaction. This somewhat exotic acronym is derived from the fact that Tkinter's main purpose in Python is to interact with a GUI system dubbed Tk. Because you could alternately just use Tk,
Tkinter is already downloaded when you install Python. To test Tkinter, do
Learn More about Tkinter
If you want to learn more about tkinter, you can take AoPS's online course Intermediate Programming with Python at https://artofproblemsolving.com/school/course/python2.