This expression looks really complicated, but it's easier to understand with a few examples:

It's hotter in Washington than it is in Los Angeles.

She's happier in the new apartment than she was in the old one.

English speakers often phrase comparisons differently:

Bethany is a lot quieter in person than over the phone.

It's hotter in Washington than Los Angeles.

But this is technically wrong. It can lead to confusion sometimes:

John's better friends with Ben than Stacy.

Does this mean that John & Ben are closer than John& Stacy? Or does it mean John & Ben are closer than Ben & Stacy? It's unclear. So it's better to say:

John's better friends with Ben than he is with Stacy.