Tyler Cowen explains it succinctly:
[P]art of the deal is that newly arriving refugees in Greece get sent to Turkey, but in return the EU takes a refugee currently in Turkey. The goal is to reduce the incentive to migrate as a refugee, since you end up in Turkey rather than in Europe.
But Cowen then points to Rachman and Oktem, who see obvious problems with the deal:
Gideon Rachman writes:
First, will the Greek authorities have the administrative capacity to process and turn around refugees arriving on their islands — as well as the many thousands already stranded in Greece? Second, will Turkey really co-operate — particularly given the fact that the EU is unlikely to deliver on all its promises? (The pledge of visa-free travel for Turks is unpopular in many EU states.) Third, will migrants desperate to get to Europe find alternative routes — perhaps via Libya, which has no properly functioning government?
Kerem Oktem summarizes the deal and makes some excellent points, including this one:
…we know that desperate people cannot be stopped. They will simply resort to new routes that will be more dangerous, more lethal and more expensive, whether it is the land borders between Turkey and Bulgaria, the boat journey from Libya to Italy or a new trajectory through Ukraine and Eastern Europe.