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.
You start unstaking 300M REM. After two months you decide to stake tokens again. 200M tokens left pending release get unlocked and staked again. If you decide to additionally stake the tokens that were previously released, the entire stake would get locked again for six months as described in the staking procedure.