A tautology refers to a statement that is true by virtue of the definition of the words in it. Because analytic statements are true by virtue of the words in them, while synthetic statements can be shown true by examining the world around us, analytic statements are tautological, while synthetic statements are not. For example, the statement "All bachelors are unmarried men" is analytic because the definition of "bachelor" is "unmarried man." By contrast, the statement "Jim is a bachelor" is synthetic as one would have to examine the world -- i.e., ask Jim -- to determine if he is, in fact, an unmarried man.
German philosopher Immanuel Kant posits that analytic processes are a priori, while synthetic processes are a posteriori. This is a fancy way of saying that we can know an analytic statement is true prior to experiencing the world, and we can know a synthetic statement posterior or after experiencing the world. By suggesting that analytic processes were a priori, Kant was arguing that the truth value of such statements exists independently of human experience and is therefore universally true according to some natural law. Synthetic processes being a posteriori, on the other hand, rely on human experiences and are thus not universal or natural.
As with Kant, philosophers such as Gottlob Frege and David Hume maintained that analytic processes represented statements with universal explanatory power, such as mathematics and logic. Such statements were believed to be free from the cultural or personal bias that trapped synthetic processes and statements, and as such, analytic statements were the only types of statements that could be believed without any uncertainty or doubt. From these conceptions of analytic and synthetic processes, Kant, Frege and Hume developed various explanatory systems relating to topics such as science, morality and aesthetics.
Analytic processes have historically been assumed to have a direct connection to reality, while synthetic processes are mediated through an individual who is himself a product of cultural and individual biases. The lack of mediation of analytic processes and statements means they are directly representational of reality and therefore truer than the indirect, mediated representations of reality that are synthetic processes and statements.