The line (or lines) between the lines beginning <<<<<<< and ====== here:
<<<<<<< HEAD:file.txt
Hello world
=======
... is what you already had locally - you can tell because HEAD points to your current branch or commit. The line (or lines) between the lines beginning ======= and >>>>>>>:
=======
Goodbye
>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
... is what was introduced by the other (pulled) commit, in this case 77976da35a11. That is the object name (or "hash", "SHA1sum", etc.) of the commit that was merged into HEAD. All objects in git, whether they're commits (version), blobs (files), trees (directories) or tags have such an object name, which identifies them uniquely based on their content.