A favicon is the icon that appears in the tab of a browser.
To add one to your in5 project, open the in5 export dialog within InDesign (File > Export HTML5 with in5...) navigate to the Advanced section.
Choose browse... under the Icon heading and choose a .png image to use as your favicon.
Now, when you click OK to export, the image you selected will appear as a favicon in the browser.
Note: this icon setting is also used for the Home Screen icon when you export a Web App.