What is a merge conflict?

In What is a branch? we presented an example of successfully merging a branch into another. However, when we try to merge one branch into another, we may find that the two branches have conflicting changes. This is known as a merge conflict.

Consider two branches that make conflicting changes to the same line of a file:

1. Replace "Second line" with "My new second line":

    First line
   -Second line
   +My new second line
    Third line
   

2. Replace "Second line" with "A different second line":

    First line
   -Second line
   +A different second line
    Third line
   

There is no way to automatically reconcile these two branches, and we have to fix this conflict manually. This means that we need to decide what the true result should be, edit the file to resolve these conflicting changes, and commit our modifications.