You need to specify the path where your chromedriver is located.
Place chromedriver on your system path, or where your code is.
If not using a system path, link your chromedriver.exe (For non-Windows users, it's just called chromedriver):
browser = webdriver.Chrome(executable_path=r"C:\path\to\chromedriver.exe")
(Set executable_path to the location where your chromedriver is located.)
If you've placed chromedriver on your System Path, you can shortcut by just doing the following:
browser = webdriver.Chrome()
If you're running on a Unix-based operating system, you may need to update the permissions of chromedriver after downloading it in order to make it executable:
chmod +x chromedriver
That's all. If you're still experiencing issues, more info can be found on this other StackOverflow article: Can't use chrome driver for Selenium