Mail merge is a feature that allows users to generate multiple personalized documents from a main document and a data source. The main document contains text and graphics that are identical for each merged document, while the data source provides unique information like names and addresses to populate each document. When the main document and data source are merged, it produces individualized documents like letters, emails, labels, or envelopes customized for each recipient listed in the data source. Common data sources used include Excel worksheets, Access databases, and other database files.