I think this is no particular truth. The poet insists that we should always be truthful whatever we say. It’s all about good human qualities or virtues. Like any other virtue, truthfulness plays very important role in the overall progress of a country. 

Truth referred to here is the truth which we have to speak if we want betterment of our society because betterment of one’s country lies in its own countrymen and countrymen can do prgress only by saying truth.