Truth is the most commonly used name for the mysterious and ethereal being that sits before The Gate and addresses alchemists who have stepped into God's domain through the use of alchemy. It appears as a pure white, nearly formless being almost indistinguishable from the rest of the endless white expanse which surrounds the Gate but for a shadowy outline which causes Truth to reflect the shape of the person it is talking to, thereby supporting its claim to be another shade of that very person. When Edward and Alphonse Elric each appeared before the Gate, Truth took on the shape of a young boy and when Izumi Curtis appeared, Truth's shape reflected that of a young woman, appeared as a vigorous man to Roy Mustang, and a white version of Father's flask form.

Truth initially introduces itself as the being some people call God, the World or the Universe, at first seeming welcoming and jovial to its guests before calling attention to their hubris and opening the Gate, which forces them inside. At this point, Truth often smiles with an eerily wide and insane grin that is similar to that of the Homunculus Gluttony and Father.

When the alchemist in question returns from his or her sojourn through the Gate, Truth informs them of the necessary toll for accessing the Universe's knowledge and collects it, usually in the form of a body part which disintegrates off the alchemist and reappears on Truth's form. In this way Truth is responsible for the taking of Edward's left leg and right arm, some of Izumi's internal organs, Roy's eyesight, and Alphonse's entire body.

It would appear that Truth's goal is to discourage humans from treading in God's domain, as it calls Edward a fool when the young boy returns and sacrifices his arm to recover his brother's soul.

Source: Fullmetal Alchemist Wiki