Hello there!

I'd recommend you to read the Steemit Whitepaper, which explains all of the terms you need to understand when using Steemit (Power Down included).

You can find it here

EDIT: From the whitepaper:
"Transferring from STEEM to SP is referred to as powering up while transferring from SP to
Steem is referred to as 'powering down.'"

I still recommended you read it though.