Planning Poker is a tool used to assist with estimating the development effort required for a story in an Agile project.
It was first described in 2002 by James Grenning and made popular by Mike Cohn of Mountain Goat Software.
The original method was for the developers to write the value on a piece of paper and show that. As they did more estimates they would end up reusing the same value and have a set of "cards".
Now actual sets of cards based on real playing cards are available.
This iPhone version replaces the cards and allows developers to estimate using their iPhone.
It works in the same way. You click on the number you want to use for your estimate. Your iPhone will display the card chosen. When everyone has come up with an estimate you all show your card.
There is also a timer so that the runner of the estimating session can constrain the discussion for each task being estimated.