Bart D. Ehrman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 120-121.
A “redactor” is someone who edits a text; “redaction criticism” is the study of how authors have created a literary work by modifying or editing their sources of information. The underlying theory behind the method is simple. An author will modify a source of information only for a reason – why change what a source has to say if it is acceptable the way it is? If enough changes point in the same direction, we may be able to uncover the redactor’s principal concerns and emphases.
We can subject the Gospels to a redactional analysis because we are convinced that their authors used actual sources in constructing their narratives; that is, they didn’t make up most of their stories themselves. Moreover, we are relatively certain that at least one of these sources still survives. To put the matter baldly: most scholars believe that Matthew and Luke used the Gospel of Mark as a source for many of their stories about Jesus. By seeing how their authors edited their stories, we are able to determine their distinctive emphases.