Nathaniel Ponder at the Peacock in the Poultrey, was publisher of the first edition of The Pilgrim's Progress by John Bunyan.
"Nathaniel Ponder" - John Bunyan had an affinity for Nathaniel Ponder. Not only was Nathaniel a publisher of nonconformist literature, but he had spent time in prison for it, much the same as John Bunyan, only for a far shorter time period.
At the "sign of the Peacock" - An identification sign (signboard) outside the business. Graphical signs were used because much of the population was illiterate.
"In the Poultry" - Poultry is a road in central London. The roads go from major intersection to major intersection only, so it was a short stretch of road (Poultry is between Cheapside and Cornhil). At some point the poultry trade was at that location.
"Near Cornhil" - Cornhil was the same road as Poultry only further down, indicating the business was down near where Cornhil started.