Mail merging in MS Word allows users to combine a standard document with recipient data from an external source like a spreadsheet or database to automatically generate personalized versions of the document for each recipient. The mail merge process involves 6 steps - selecting the document type, choosing recipients from an existing list or contacts, writing the document and inserting personalized fields, previewing the results, and finishing the merge to generate the final documents or emails. Mail merging is commonly used to create personalized letters, labels, emails or other documents for a list of recipients.