Mail merge is a process that uses a template to send personalized emails to multiple recipients, efficiently managing large volumes without manual copying and pasting. This blog post details how to create a mail merge template using Gmail as the email service provider and Ruby programming language, supported by the Nylas Email API. The process involves setting up a .csv file containing recipient details such as names and email addresses, which are then used to replace placeholders in the email template for personalization. The guide outlines the installation of necessary Ruby packages, including Sinatra, and provides a step-by-step approach to implementing the mail merge application, from setting up the environment to executing the script that sends personalized emails to recipients. The application is tested by running it locally on a server, demonstrating the successful sending of emails with personalized content.