This phrase is usually used in Legal matters such as giving evidence in a court of law. This is part of the oath you must swear to to tell the truth and only the truth (means not to tell any lies..) cheers

This is often found in courts as part of the phrase, "I swear to tell the truth and nothing but the truth" when a witness raises his right hand and puts his left hand on the Bible. It means that the person promises not to lie about facts in a court case.

Truth = Be honest and true with words. "The truth and nothing but the truth"= Being honest, and not being misleading or giving false information.

it means to tell the truth. no lies, just the truth.