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.