Bootstrap's grid system is arguably its most impressive and most commonly used feature, as it solves the important task of horizontal and vertical positioning of a page's contents, allowing the contents to be structured across multiple display widths. Therefore, mastering this core functionality is essential for any Bootstrap developer.
As already noted in Chapter 1, Revving Up Bootstrap, Bootstrap 4 is mobile-first. As such, it should come as no surprise that the grid system is optimized for smaller viewports, and it scales up to suit larger viewports (as opposed to scaling down to smaller viewports).
Note
What is a viewport?A viewport is the available display size to render the contents of a page, that is, it refers to your browser window, minus the toolbars and scrollbars. The viewport should not be confused with the viewport meta tag, which is used to define the viewport's actual display size. To elaborate, as already noted in Chapter 1, Revving Up Bootstrap, mobile devices...