The Hive savings account is located in one's wallet. This is an option that allows one to move $HIVE and the Hive Backed Dollar (HBD) from liquid form. Removing either coin is subject to the 3-day withdrawal period.
Users can opt to place HBD in savings to earn a return. At present, any HBD submitted pays 20% interest annually (APR). This is a rate that is set by the consensus of the witnesses.
There is no interest paid on $HIVE placed into savings.
The savings account is a base layer security measure. If an account is hacked, the withdrawal period does prevent the nefarious individual from removing the money. This idea is to allow the account holder time to reset the private keys to prevent further damage.
$HIVE placed in savings does not factor into on-chain governance. It is only through the powering up, or staking, process that $HIVE becomes Hive Power. There is also no impact upon curation rewards.