You can get access to Display object through the Map.Paint event where it's available as a MapPaintEventArgs.

The Display class allows you to draw graphics, text, or images on map control for displaying purpose or as a way to provide additional information to end users, such as a customized text showing map title, drawing feedback on screen following user's mouse movements, or showing a company logo.

Please note that elements (graphics, texts, images) placed on Display won't persist, which implies that developer needs to take care of keeping these elements and redrawing them again when the map gets re-painted.

Also note that the Display class will draw elements after all other layer drawings had been completed. The Map component will draw layers from mobile cache first, then any MapGraphicsLayer (if available), and lastly the elements drawn by Display class.

The Display class provides a full set of methods to draw various types of elements on Display. Among them, the DrawGeometry method will enable you to draw a geometry with customized pen and brush, while DrawMapImage draws a MapImage object on the Display. As a commonly used method, DrawText will place a customized text on screen, and the MeasureText method allows for measuring of the specified string when drawn with the Font object you specified.

The following methods draw various types of graphics: DrawPoint, DrawMultipoint, DrawLine, DrawPolyline, DrawPolygon, and DrawEnvelope.

System.Object
   ESRI.ArcGIS.Mobile.Display

Reference