What kind of widget is it?

[DT2000]

I believe what you are looking to use would be the ttk.Separator.

import tkinter as tk
from tkinter import ttk

root = tk.Tk()
# PADX aligns the Separator clearance from edge of left side and right side
PADX = 5

toolbar = tk.Frame(root).grid(padx=100, pady=10)

button1 = tk.Button(toolbar, text="Home").grid(padx=10,pady=10)
sep = ttk.Separator(toolbar, orient=tk.HORIZONTAL).grid(padx=PADX * 2, pady=10, sticky=tk.EW)
sepL =tk.Label(toolbar, text="   Using a 'ttk.Separator' widget").grid(padx=PADX, sticky=tk.W)
button2 = tk.Button(toolbar, text="Insert").grid(padx=50, pady=10)

root.mainloop()

I hope this helps and points you in the right direction.