Transferring a domain away from Sav allows you to move your domain to a different registrar.
When your domain expires, it enters into an Expired Renewal Grace Period which lasts 30 days at Sav. During this period, the domain can still be renewed or transferred. While your domain is in the Expired Renewal Grace Period, it may or may not be disabled. This varies depending on the TLD.
When the Expired Renewal Grace Period ends, the domain goes into a 10 day Expired Auction. Once it is in that stage, you can no longer transfer it. If no one buys the domain after those 10 days, it goes back to the registry and no one can do anything to it until it is released.
We recommend always renewing your domains before they expire to prevent them from being deleted or auctioned off.