Something tautological is redundant and circular, especially when talking about logic. "Logical things are logical" is a tautological sentence.
We use the word tautological for statements that go in a circle, like this sentence: "A circle is circular." This is a logical problem, because it doesn't make sense to define something in terms of itself. If you don't know what a fish is, then "fish are fishy" means nothing. When you're trying to make a point, avoid tautological statements.