Cancellation of Sprint

Hi all,

I have some difficulty in understanding the following statement. Hope to get some help.

"When a Sprint is cancelled, any completed and “Done” Product Backlog items are reviewed. If part of the work is potentially releasable, the Product Owner typically accepts it. All incomplete Product Backlog Items are re-estimated and put back on the Product Backlog. The work done on them depreciates quickly and must be frequently re-estimated."

What does the `the work done on them depreciates quickly and must be frequently re-estimated` mean?

Thanks in advance