An account may decide to unstake tokens (not permitted during the lock period over the first six months of staking). After this, staked tokens will gradually start to release over a period of six months on a weekly basis. This is done to secure the network from speculative behavior among Guardians and BPs.
For example, an account with 300M staked tokens starts to unstake. After 2 months they decide to stake tokens again. There were 200M tokens left pending release so those tokens come back into the staked and unlocked state. If the account decides to additionally stake the tokens that were previously released, the entire stake would get locked again per the locking formula described earlier.
What's next: Restaking unreleased tokens