It means we've borrowed the css directly from Bootstrap for the grid (as well as some variables and mixins) so, aside from giving credit, if you are familiar with the bootstrap grid it should be easy to get up and running with Elemental.

We're actually developing it first and foremost for use in Keystone's Admin UI - we needed a robust and flexible css + component library for that, which users could extend & build plugins on, so we started building one.