Marketo Sales Insight is a tool that provides salespeople with insights about leads and customers from their activity on a company's website and in email campaigns. It integrates this data into the salesperson's view in Salesforce. The document discusses how to set up Sales Insight by installing the app in Salesforce and configuring settings. It also explains how to create "interesting moments" like webinar registrations or page visits that will be displayed to salespeople. The document provides examples of how to trigger interesting moments from certain actions and use tokens to populate details. It also discusses options for exposing emails and campaigns to Sales Insight so sales can interact with marketing assets.