Adding Module chrome allows us as template developers to control the output of a modules HTML and Css, so it fits with our template. With Joomla 3+ we are able to use Bootstrap core functionality to render Module Chrome in several ways (depending on the template)

Module chrome on Joomla Bamboo templates can include

zendefault - A default module chrome
zentabs - a tabbed interface
zenslider - use the module title to slide open or closed a module
zenmodal - Use the module title to trigger a modal window with the module content in the modal window.
zenpopover - Use the module title to display a tooltip / popover with the module content.

To assign Module Chrome to a module you select options > advanced options and select from the module style drop down.

The code for Module Chrome can be found in the template html folder for example - \templates\grid3\html\modules.php

This doc talks about module chrome as well - docs.joomlabamboo.com/zen-grid-framework-4/theme/applying-module-chrome