An emendation is a correction made to a manuscript. There are all sorts of motives which would lead a scholar to suggest such a change. He may believe that his change is a reading more closely aligned with the original text. He may think his change makes better sense. Some times emendations are made to support a given theological point or a preconceived understanding of what the text must mean. Evangelical scholars typically reject emendations as much as possible. An example is the interpretation of Shiloh in Genesis 49:10 which almost all scholars emend in some fashion.
See also ketiv.