It's the President's authority for spending the funds without legislation. Though it couldn be spent on anything, my guess we simply cut a check to UNWRA. This was announced with some fanfare a week or so ago, note the pic of George Mitchell a few posts up making the anouncement at an UNWRA warehouse in Jerusalem.
One interesting aside, UNWRA has ceased delivering supplies to Gaza due to thefts by Hamas. Though I'm confident they'll get their $ from the US, I wonder if the Gazans will get any aid.
Like I said, I hope you are right. Palestinians get nearly $8 billion a year in aid. $20.3 million for migration assistance is either too small to be useful or it means just that. Migration assistance. I do hope you are right.