Now I Get It: White House security clearance

President Trump's son-in-law and White House senior adviser Jared Kushner has had his security clearance downgraded from top secret to secret level. He's not the only one, as chief of staff John Kelly ordered that dozens of other White House officials with interim clearances pending before June 1, 2017, will lose their access if they didn't get permanent clearances by last Friday. What is a security clearance and what are the different levels?