What is it for something to be true?

Trivial answers would be that true things are facts, or true things have been proven, or that the something is true is something actual. These are trivial answers, lacking the depth to explain, they only restate.

Religious fundamentalists would say that the truth is the word of God.
A casual atheist would say something true is something proven by science.

Therein lies a problem, if either of those answers are yours. Truth, as it is properly spoken of by science rests on the world. The truth describes a situation that corresponds to reality. This sounds very trivial, and it really ought to be, but since most people don’t consider this they can be led astray on discussions of truth. The truth is not the truth because it is what scientists claims, or it is what evidence shows. Evidence and the claims of professionals are reasons to believe that the world is such a way. Science does not make the facts of science true. All the true facts of science are already true, and have always been true, whether discovered or not. It ought to be the same with religion, it is not a God’s saying something that makes it truth, but that the statement agrees with the state of the world that makes it true. Though the testament of a reliable figure is a reason to believe something, it being said or written does not produce the truth.

For all his prejudice and awful metaphysics, here I agree with Schopenhauer:

Truth does not depend on any favor or disfavor, and does not have to ask anyone’s permission; it stands on its own two feet; time is its ally; its power is irresistable; its life indestructible. (82)
[Philosophical Writings, Arthur Schopenhauer, translated by E.F.J. Payne  The German Library, Continuum publishing, 1994.]

Truth is not a quality, something is not true like it is big, or like it is red. Truth is a special relation between the world and a statement.