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Bootstrap is an open source toolkit (developed by Twitter but usable by anyone) for developing web sites with a standardised layout and which work nicely on a variety of screen sizes (e.g. mobile, tablet, computer).
Both Joomla and Fabrik support Bootstrap as their primary layout. It is not mandatory for you to use it on your web site, but if you do then you need to make sure that your front-end template and Fabrik are based on the same version of Bootstrap (2, 3 or 4).
Joomla 3 standardises on Bootstrap 2 (which is pretty dated now), however you can get templates (like Helix III or Purity 3) which are based on Bootstrap 3. (The future Joomla 4 will be standardising on Bootstrap 4.)
Fabrik 3.x is also based on Bootstrap 2, so if you are using a front-end template based on Bootstrap 3, then you need to apply some Fabrik template overrides from https://github.com/Fabrik/Fabrik-Joomla-alt-layouts
If you don't want to use Bootstrap, then in Fabrik you should use the Table template, and you don't need to worry about what your front-end template supports.
If you do want to use Fabrik's Bootstrap template then you need to make sure that either:
a. your template is a bootstrap template - like Joomla 3's standard front-end template Protostar; or
b. installing a Joomla extension like this to include bootstrap when your template doesn't.
This is particularly true if you are using Tabs in your list or Bootstrap_Tabbed form or details templates as the tab functionality is provided by the bootstrap functionality.