This is when one converts $HIVE to Hive Power. It is staking on the Hive blockchain.
Powering up Hive increasing one's voting power for both governance issues along with distribution of the reward pool. The amount of Resource Credits one has available increases as HP increases.
Stake is what determines what one will receive when voting content. Many grow their HP so as to be valuable curators on-chain.
Powering up $HIVE is a commitment to the ecosystem since the powering down process requires 13 weeks to full release the money. Each week, 1/13th of the stake is delivered in liquid $HIVE.