A modal is a dialog window which can be displayed over the current page.

<!-- Clicking the button will open the modal window -->
<button type="button" class="btn btn-success btn-lg" data-toggle="modal" data-target="#theModal">Open The Modal</button>

<!-- The Modal -->
<div id="theModal" class="modal fade" role="dialog">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <button type="button" class="close" data-dismiss="modal">&times;</button>
                <h4 class="modal-title">Text For The Modal Header</h4>
            </div>
            <div class="modal-body">
                <p>Text for The Modal Body.</p>
            </div>
            <div class="modal-footer">
                <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
            </div>
        </div>
    </div>
</div>