This references the way the layout of the content of the page changes at specific breakpoints. For example, in our mobile sizes, usually our content lines up under each other for a one column layout. As the page gets wider, the content may shift to a two-column layout and so forth.
Quote:
The web’s moved beyond the desktop, and it’s not looking back. The number of devices we’re designing for is growing just as quickly as mobile traffic. — Ethan Marcotte