i saw this code in one of the tutorial of bootstrap.
In which presenter change the size of browser and content of html get change.Reflecting the same on my system but not able to understand what exactly is going on.
Classes do not come into and out of "existence" when the browser width changes. They are always there and present on the elements upon which they are declared. What changes is how those classes are applied in the CSS, usually via CSS media queries ⇐ I believe this is the concept you need to research.