The Bootstrap grid is automatically responsive. So long as the grid and its containers are used properly, your layout will become responsive whenever the
.sm, .md, .lg breakpoints are hit.
Upon hitting said breakpoint, the Bootstrap grid columns stack and take the width of the container, which is usually 100%. Form fields and in some cases buttons (if specified) will also
display: block; and span 100% width of the layout.
Please see the JSFiddle in the Demo section for a working example of this.