This is a tricky concept to grasp when hearing about it for the first time. I'll again point out that the same clip can be found on my YouTube channel, Science 360 - Beyond the Sky. It contains visuals that may make it easier:
https://youtu.be/HkNTA7zX-u0

Be sure to check out the inquiry questions below that video.

I'd also like to point out that the discovery of the Cepheid Variable star is generally attricbuted to Henrietta Swan Leavitt. This audio I did made it sound like Hubble made the discovery. He did USE them but only because of the work of Leavitt.