Apart from the Text widget, our application contains three buttons that call the methods to clear the whole text content, insert the "Hello, world" string in the current cursor position, and print the current selection made with the mouse or the keyboard:

import tkinter as tk

class App(tk.Tk):
    def __init__(self):
        super().__init__()
        self.title("Text demo")
        self.resizable(0, 0)
        self.text = tk.Text(self, width=50, height=10)
        self.btn_clear = tk.Button(self, text="Clear text",
                                   command=self.clear_text)
        self.btn_insert = tk.Button(self, text="Insert text",
                                    command=self.insert_text)
        self.btn_print = tk.Button(self, text="Print selection",
                                   command=self.print_selection)
        self.text.pack()
        self.btn_clear.pack(side=tk.LEFT, expand=True, pady=10)
        self.btn_insert.pack(side=tk.LEFT, expand=True, pady=10)
        self.btn_print.pack(side=tk.LEFT, expand=True, pady=10)

    def clear_text(self):
        self.text.delete("1.0", tk.END)

    def insert_text(self):
        self.text.insert(tk.INSERT, "Hello, world")

    def print_selection(self):
        selection = self.text.tag_ranges(tk.SEL)
        if selection:
            content = self.text.get(*selection)
            print(content)

if __name__ == "__main__":
    app = App()
    app.mainloop()