Stands for Hive Power Up Day.
This is an event that is held the first of each month. It is designed to bring awareness of the importance of staking of $HIVE. The members of the Hive blockchain each take the opportunity to move liquid coins and convert them to Hive Power. This is the term used for staked coins on-chain.
Increasing one's Hive Power affects the voting power, which translates into rewards for both curator and content creator. It also helps to remove $HIVE from the floating supply. Curation using voting power is how the reward pool is distributed.
The event is marked with prizes and badges for those who participate.
Individuals on Hive who participate often follow up by writing posts about what they did. This helps then to earn greater rewards as many are involved in supporting the others.
These are activities that are becoming very familiar in the world of cryptocurrency.
HPUD inspired other events similar such as Leofinance's LPUD.