This item is one of the most powerful objects in all of comic books.
The Lasso of Truth is an extension of Diana's belief in the truth. It is always at her side or in her hands, yet she never is concerned about it, as she lives her life in truth so she has nothing to fear. The Lasso is Truth, and that truth can be as big or little as she needs, and is unbreakable (even to the point of being able to bind its wielder) as that which is true can never be anything but what it is. It can compel truths from liars, but also control the will of those who don't believe in the strength of their own person-hood.
And yet it is interesting how seldom it is used to extract the truth, because what need does Darksied or even Lex Luthor have of lies when they can strike with devastating truthfulness of their own?