Less is a dynamic stylesheet language and a great tool for front-end developers. It saves time and makes modification and maintenance of stylesheets much easier. Detailed information about Less CSS can be found at lesscss.org, but it's not a requirement. You can use regular, old CSS in .less files or you can simply just use CSS. The choice is yours.
MuraBootstrap is a Mura-specific implementation of the robust Bootstrap front-end framework. If you're not familiar with Bootstrap, we highly recommend having a look at it as it can also be a great tool to have in the toolbox. You can learn more at www.getbootstrap.com, but you'll gain some familiarity simply by using it within Mura CMS.
NOTE: It is not a requirement to use the Bootstrap framework to use Mura CMS, you can use any framework you like or none at all. It's entirely up to you how you like to develop in Mura CMS.