In the example below there are three elements, neatly placed next to each other. When the page width decreases, the elements first shrink. This starts when the width falls below 1000px.
(As do these paragraphs...)
Then, at 850px, element 3 stacks below the other elements. Now each element has more room to display its contents. At 525px the elements 1 and 2 stack as well.