Selenium webdriver wait for ajax with Python

Selenium webdriver wait for ajax with Python

from selenium import webdriver
from selenium.webdriver.support.ui import WebDriverWait
from selenium.common.exceptions import WebDriverException
 
def ajax_complete(driver):
    try:
        return 0 == driver.execute_script("return jQuery.active")
    except WebDriverException:
        pass
 
def my_automation_test():
    ff_driver = webdriver.Firefox()
 
    ff_driver.get("http://domain.tld")
 
    #wait for ajax items to load
    WebDriverWait(ff_driver, 10).until(
             ajax_complete,  "Timeout waiting for page to load")
 
    assert "ajax loaded string" in ff_driver.page_source

Why?