Hi, It's not currently possible to label your features using Explorer Online. If you'd like to see this added please submit this request to the ideas website under the ArcGIS Online section.
As a workaround you could confirgure the popup Title to be the field that you were interested in labelling by. That way when you mouse over the feature you will see the attribute displayed. To do this go the the layer's details (right arrow beside the layer name in the Contents panel), then click the Pop-up Configure button then set the Title to be the field of interest.
