Field codes are used in Microsoft Word if you use bibliographic software (like EndNote) to create your references.
You can see that you have field codes when you click on the bibliography and Word highlights all references grey:
In the final version of the manuscript you need to remove these, because they are hard to copyedit and lead to problems with the RefCheck process after acceptance (see What is the RefCheck process?).
Before re-submitting your revised manuscript, open your document in Word, select Tools -- Endnote (or Reference Manager) -- Remove Field Codes, and save the manuscript under a new name. Then resubmit that version.
Removal of field codes
If your used a reference management system (like Endnote), in-text references (and the bibliography) may appear as field codes (can be recognized by a grey background). In this case, make sure to remove all field codes and hidden links from the manuscript immediately after acceptance and before copyediting.
Before re-submitting your revised manuscript after acceptance, open your document in Microsoft Word, select Tools -> Endnote (or Reference Manager) -> Remove Field Codes -> save the manuscript under a new name. Then resubmit that version.
To remove hidden links, select all text (Ctrl+A or Cmd+A), then press Ctrl+Shift+F9 or Cmd+6 to unlink all fields.
Using these methods, your in-text citations and bibliography should become regular text, without field codes or any hidden links. Save under a new file name and upload this as your final manuscript version.