Click on the map to get a popup. The popup is composed of a few basic elements: a container, a close button, a place for the content, and an element to represent the little arrow at the bottom. To anchor the popup to the map, an ol.Overlay is created with the popup container. A listener is registered for the map's click event to render the popup, and another listener is set as the click handler for the close button to hide the popup.

See the popup.js source to see how this is done.