Eliyah Kohein19,328 Points
It's an associative array of objects. $data is an object. If the question is why is there a $ in front of $field when calling the property it's because we are using the value of the passed $field perameter as the property name.
So, if the value of $field = 'name'.
$data->$field is == $field->name.
Steven Parker170,483 Points
That's the PHP notation for a member of an associative array. The term on the left is the key, and the one on the right is the value.
You're in an intermediate-level course, which assumes a basic familiarity with the language. You might benefit from setting it aside and taking some of the fundamentals first. In particular, this notation and its use are covered in the PHP Arrays and Control Structures course.