This cordoned off central section of St. Peters is probably for a special procession for All Saint's Day.
It leads the eye in this image to the Baldechino through which can be seen the Cathedra Petri in the apse.
Gianlorenzo Bernini was the designer and maker of both the Baldechino and Cathedra Petri when he was
the chief architect and designer of the interior of St. Peters.

The basilica was originally designed to be a Greek Cross shape, (four equal arms), but during the Cournter Reformation of the Catholic Church,
the church demanded longer naves to accommodate large crowds, so the architect Carlo Maderno extended the nave by three
bays to conform to the new Counter Reformation standards, turning the original Greek Cross floor plan of four equal arms
into a Latin Cross, lengthening the lower arm of the cross.
This almost invisible extension can be seen when one is inside the nave and looks up at the bays.