Bootstrap is a front end framework (i.e. the user interface) for web applications and websites. It combines HTML and CSS with optional JavaScript extensions to create design templates for forms, buttons, navigation, and layouts. Bootstrap 3 is designed to be responsive and mobile first, meaning that it is designed by default for use on small devices like phones and tablets and scales up for use on larger screens.
LibGuides v2 is built using Bootstrap 3, so your LibGuides site is mobile friendly right out of the box. You can customize many LibGuides elements by providing additional CSS rules. Bootstrap elements can be added by assigning Bootstrap classes, without any additional coding.
(SpringShare, Training: https://training.springshare.com/libguides/cms-library-websites/bootstrap-media-queries)