The eight limbs of the yoga called “Ashtanga Yoga,” are delineated in the Yoga Sutras of Patanjali. The first limb of yoga, called yama, set forth in sutra 2.30, is composed of five restraints: ahimsa (non-harming), satya (truth), asteya (non-stealing), brahmacharya (walking in God), and aparigraha (not grasping at materialism).
Ahimsa is the central practice. All of the others have to be practiced with ahimsa.
So, the second yama, satya, truth, means that one aims to tell the truth so long as that truth is not harmful. If telling someone the facts would bring them harm, then ahimsa is violated, and that “truth” is not considered to be satya.
Remember, the yamas are yoga practices. It’s not that everyone should practice this way, but if you’re a yoga practitioner, then this is an essential practice.