This methods wraps the text of the first parameter into the tags specified by the other two parameters.
IN ASCX: <%# SearchHighlight("Hello World!","<b>","</b>")%>
In Text/XML you would need to create a simple custom macro where in you back-end code call the exact same method:
CMS.DocumentEngine.Web.UI.TransformationHelper.SearchHighlight(param1, param2, param3)
But to wrap a text? you can do it with simple format string:
{%String.FormatString("<b>{0}</b>",", "Hello World!")|(identity)GlobalAdministrator%}