selenium not running firefox

[Sanlus]

so i was trying out a simple code like this:

from selenium import webdriver
browser = webdriver.Firefox()

and it returned me a huge error message:

Error:
Traceback (most recent call last):
  File "C:\Users\MSI\AppData\Local\Programs\Python\Python37-32\lib\site-packages\selenium\webdriver\common\service.py", line 76, in start
    stdin=PIPE)
  File "C:\Users\MSI\AppData\Local\Programs\Python\Python37-32\lib\subprocess.py", line 756, in __init__
    restore_signals, start_new_session)
  File "C:\Users\MSI\AppData\Local\Programs\Python\Python37-32\lib\subprocess.py", line 1155, in _execute_child
    startupinfo)
FileNotFoundError: [WinError 2] The system cannot find the file specified

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\MSI\AppData\Local\Programs\Python\Python37-32\Automate the Boring Stuff with Python Course\seleniumwebdriver.py", line 2, in <module>
    browser = webdriver.Firefox()
  File "C:\Users\MSI\AppData\Local\Programs\Python\Python37-32\lib\site-packages\selenium\webdriver\firefox\webdriver.py", line 157, in __init__
    self.service.start()
  File "C:\Users\MSI\AppData\Local\Programs\Python\Python37-32\lib\site-packages\selenium\webdriver\common\service.py", line 83, in start
    os.path.basename(self.path), self.start_error_message)
selenium.common.exceptions.WebDriverException: Message: 'geckodriver' executable needs to be in PATH. 

so what does this say? and what do i have to do to fix it?