Every Holy Week, we hear Pontius Pilate's question to Christ, "What is truth?" The answer you can give matters.
There are several theories to answer the question: The "correspondence theory" says that truth is when your mind (or intellect) corresponds to the world. The "coherence theory" says that truth is when a statement you make coheres to an accepted system of other statements. Finally, the "pragmatic theory" says that if what you believe is practical (i.e., it works), then it is true.
Aristotle acknowledges that each of these theories has some truth but is also incomplete.
He offers his own explanation of truth: that it is the objective world, or reality, that is the measure of truth. Truth does not originate in our minds. Man is not the measure of truth. Rather, that which is is truth (being is truth). The world is true. Our thoughts are true when they express the reality of what is.
That expression of being is the logos (or, "word"). Aristotle thinks that it is not private ("I can only speak for my own experience of the logos"). Rather, every mind can by its nature experience the same logos (the "word" of being) and so every mind can know truth, goodness, and beauty. The world is not private, but is common.
[Note for the interested: The Logos, which is a common title for Christ, is the perfect expression of the truth of being. Later on, in St. Augustine and St. Thomas Aquinas's thought, we will see the connection the Christian tradition makes between Aristotle's logos and being and the Christ and Father the Church worships.]