PySimpleGUI is a Python package that enables Python programmers of all levels to create GUIs. You specify your GUI window using a "layout" which contains widgets (they're called "Elements" in PySimpleGUI). Your layout is used to create a window using one of the 4 supported frameworks to display and interact with your window. Supported frameworks include tkinter, Qt, WxPython, or Remi. The term "wrapper" is sometimes used for these kinds of packages.
Your PySimpleGUI code is simpler and shorter than writing directly using the underlying framework because PySimpleGUI implements much of the "boilerplate code" for you. Additionally, interfaces are simplified to require as little code as possible to get the desired result. Depending on the program and framework used, a PySimpleGUI program may require 1/2 to 1/10th amount of code to create an identical window using one of the frameworks directly.
While the goal is to encapsulate/hide the specific objects and code used by the GUI framework you are running on top of, if needed you can access the frameworks' dependent widgets and windows directly. If a setting or feature is not yet exposed or accessible using the PySimpleGUI APIs, you are not walled off from the framework. You can expand capabilities without directly modifying the PySimpleGUI package itself.
Bridging the "GUI Gap"
Python has brought a large number of people into the programming community. The number of programs and the range of areas it touches is mindboggling. But more often than not, these and technologies are out of reach of all but a handful of people. The majority of Python programs are "command line" based. This isn't a problem for programmer-types as we're all used to interacting with computers through a text interface. While programmers don't have a problem with command-line interfaces, most "normal people" do. This creates a digital divide, a "GUI Gap".
Adding a GUI to a program opens that program up to a wider audience. It becomes more approachable. GUIs can also make interacting with some programs easier, even for those that are comfortable with a command-line interface. And finally, some problems require a GUI.