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.

The recommended size is 129px by 129px.

Note: this icon setting is also used for the Home Screen icon when you export a Web App.