This is the time it takes to convert ones full holding of Hive Power to liquid $HIVE on the Hive blockchain.
Another way of phrasing this is the unstaking period.
When $HIVE is staked, a process called powering up, it is locked in for a total of 13 weeks. If one does opt to power down (unstake), 1/13th the total of Hive Power will be allocated each week. This will be placed in the account wallet in the as liquid currency.
This is viewed by many as a security feature. If an account is accessed by a nefarious entity, the stake is protected since it is not in liquid form. Even if the hacker does start the process, the time it takes to release all the money gives the account owner the ability to stop the process and secure the private keys.
It also should serve as a deterrent to the centralized exchanges. If they want to use customer money to insert themselves in governance issues, as with the case with Justin Sun's takeover of Steem, then there is a penalty for that, i.e. the funds are locked up for an extended period of time.
Another security measure that was implemented as a result of that action is the 30 Day Governance Staking Period which withholds newly staked Hive Power from being applied to governance votes for a period of 30 days.