How does extjs add a prompt to a column

<script type="text/javascript"

src="${pageContext.request.contextPath }/extjs/examples/simple-widgets/qtips.js"></script>

renderer : function(value, metaData, record, rowIndex,
colIndex, store) {
// provide the logic depending on business rules
// name of your own choosing to manipulate the cell
// depending upon
// the data in the underlying Record object.

// metaData.css : String : A CSS class name to add
// to the TD element of the cell.
// metaData.attr : String : An html attribute
// definition string to apply to
// the data container element within the table
// cell (e.g. 'style="color:red;"').
metaData.attr = 'ext:qtip="' + value + '"';

return value;
}