Mail merge allows sending personalized letters to multiple recipients by merging a main document with an address data source. The main document is the letter template. The data source contains contact fields like name, address, etc. for each recipient. To perform a mail merge: 1. Create the main letter document and insert merge fields for personalization. 2. Create or select an address data source with contact records. 3. Use the mail merge wizard to link the data source to the main document, preview merged letters, and print finalized documents for each recipient. Mail merge saves time by automating letter personalization for a contact list.