Track Changes are a feature in Microsoft Word used by editors when making comments, queries, suggestions, insertions and deletions within your working manuscript. The edit is done in this way so that you, the author, can open up your edited manuscript and see exactly what changes have been made. You can then decide whether to accept or reject each change. Before resubmitting your revised manuscript for design, it should be clear of all editorial markup, meaning that all changes have been addressed by accepting and/or rejecting them.
Here is a useful resource with an overview and instructions outlining this feature: https://edu.gcfglobal.org/en/word2010/reviewing-documents/1/