A layer in your map uses a picture symbol with a reference scale. Your map will correctly display picture symbols at a fixed (reference) map scale. This portrayal of picture symbols will differ slightly from the standard ArcMap display because there are slight differences in picture symbol rendering within ArcMap and in basemap layers or map services.
This warning message is telling you that symbols in your map service or basemap layer display will differ slightly from your ArcMap data frame's display. Map rendering in basemap layers and map services is correct. However, it will differ slightly from your standard ArcMap display.
This display difference only occurs when you have set a reference scale, which fixes the symbol size to draw at the proper height and width (typically expressed in points) at the reference map scale. This will cause your symbols to become larger as you zoom in from the reference scale and to shrink as you zoom out. It's important to recognize this behavior because it is not typically what web map users want in their online, multiscale basemaps. It is likely that you will want to disable this reference scale behavior. See Working with reference map scales for more information.