Questions about an article I read from C.A.R.

[Brandon Snyder]

I had read an article from C.A.R. about mortgages, and I wanted to ask a few questions about what I read.  You can read the whole article here:

https://www.mba.org/2017-press-releases/january/mortgage-applications-decreased-over-two-week-period-in-latest-mba-weekly-survey

What does seasonally adjusted basis mean?

In the article, it reads:

"The Market Composite Index, a measure of mortgage loan application volume, decreased 12 percent on a seasonally adjusted basis from two weeks earlier"

Also in the article, it reads:

"The seasonally adjusted Purchase Index decreased 2 percent from two weeks earlier. The unadjusted Purchase Index decreased 41 percent compared with two weeks ago and was 1 percent lower than the same week one year ago."

What is the purchase index? Is it simply a measure of how many properties people are buying? Why is there such a large discrepancy in the two indexes, and why are they both relevant/significant?

Thank you in advance for your attention.