Second Yama of Ashtanga Yoga
Satya is truthfulness. It is more than just telling the truth. One's actions should be in accordance with one's words and thoughts. God and man's true Self are truth, and in order to tune in with that consciousness we need to live truthfully at all times. Furthermore lying creates many thoughts in the mind which go against the raja yoga objective of calming the mind.
Truth is righteousness. Righteousness is light, and light is bliss. Ahimsa, Brahmacharya, purity, justice, harmony, forgiveness, peace are forms of truth... Truth stands even when there is no public support. Swami Sivananda, Bliss Divine
Depending on one's inclination, there can be different conceptions of the truth. A moralist cares for the results of the truth so that if a lie must be told in order to bring about the greatest good for the greatest number of people, then this can only be the truth. It is easy to fall into this pitfall thinking that you are helping many people when in fact you may be arranging circumstances to benefit only yourself. One must be always watching the mind to be sure where one's true motive's lie.
The conception of truth for a spiritual man is that Brahman is the only absolute truth and everything else is ultimately an untruth. Many uncompassionate people can use this idea as a way to justify their actions when in fact they are really hurting a number of people with their untruths.
When one's thoughts, words and actions are truthful one will have a harmonious feeling and be free from uneasiness and the feeling that something is going to go wrong. Untruths affect the subconscious mind, and besides creating disharmony in this life, this habit will be carried to the next life where one will undergo suffering as a result. One lie can lead to another lie until a habit is formed which can affect many lifetimes.
Speaking truth is the most important qualification of a Yogin. Swami Sivananda
In this modern age, it is commonly accepted for people to commit to various things and to have no intention of keeping their word, the feeling being it is more important to tell someone what they think they want to hear rather than simply telling the truth. Swami Sivananda instructs us that it would be better to say something like, "I'll try, let me think it over," so that you don't make commitments you can't keep.
While telling the truth it is important to speak in a positive way so as not to cause hurt, yet at the same time be as straightforward and sincere as possible. To many people it might seem impractical or too difficult to always be truthful, but if practiced over time it becomes easier and many stumbling blocks along the path are removed.