Saturday, June 14, 2014 EST
Probability of Assignment (POA) is useful to determine if the an option will expire worthless. 

Keep in mind this is called "Probability" of Assignment and is not a guarantee the option will expire worthless, but another factor an option trader can use in their analysis.

How POA is Determined: POA in the Option Screener is based on the options Delta. The Options Delta is an acceptable proxy for Probability of Assignment. 

For more information on this topic click this link to review a WikiPedia article.