This mixin is based on work by Hugo Giraudel http://www.sitepoint.com/managing-responsive-breakpoints-sass/. It uses Sass Maps http://sass-lang.com/documentation/file.SASS_REFERENCE.html#maps, interpolation http://sass-lang.com/documentation/file.SASS_REFERENCE.html#interpolation_ and an if..else to convert the breakpoints you enter into media queries.
You don't have to know how the mixing works exactly (although the article goes into some explanation), but you do have to know how to modify it to set your own breakpoints. Lines 10-15 show how that is done. This technique is nice in that it allows you to decide if you are going to use min-width, max-width or a combination of the two. Alternatively you could set what is given in ems here as variables and use those.