It means that, in order to send a note, your client must do some computational work, and present as evidence a proof with your note. It has potential to reduce spam, since it introduces a cost to sending notes. That cost is measured in computational power.
Discussion
It's optional like most nips
What does Pow 2 or 3 or 4 means?
Probably has to do with the amount of computational power put into the note. Potentially the number of leading zeros in a hash somewhere. I assume it can be interpreted as higher numbers = more work.
Could let clients set a threshold to only see notes with some amount of work attached.
