This message is informational. When your input features are not projected (that is, when coordinates are given in degrees of latitude and longitude), or when the output coordinate system is set to a geographic coordinate system, or when you specify an output feature class path to a feature dataset that has a geographic coordinate system spatial reference, distances will be computed using chordal measurements. Chordal distance measurements are used because they can be computed quickly and provide good estimates of true geodesic distances.
Be sure to project your data if your study area extends beyond 30 degrees. Chordal distances are not good estimators of geodesic distances beyond about 30 degrees.