It is when one converts Hive Power to $HIVE. This is unstaking on the Hive blockchain.
Powering up $HIVE increasing one's voting power in both governance issues along with distribution of the reward pool. The amount of Resource Credits one has available increases as one powers up $HIVE.
By powering down, the entire process is reversed. Voting power decreases while the ability to engage with the blockchain is reduced since Resource Credits decline.
The power down time on Hive is 13 weeks. Each week, 1/13th of the total is delivered to one's wallet in the form of liquid $HIVE. A lengthy power down time is viewed by many as a security feature. Like with coins in the savings account, the ability to remove any HP in the event of a hack is impossible. The time allows one to reset his or her private keys.