Tautology: An expression that says the same thing twice within itself, but in a different way. Sometimes this can be an unintended redundancy or unnecessary repetition, and other times it can be a strategic use of language (often for emphasis).
Let’s suppose I told you, “I am a fanatical F.A.C.E.” The word “Fanatical” is already included in the acronym “F.A.C.E.”, so that sentence is basically “I am a fanatical Fanatical Aux Cable Enthusiast”. Whether intentional or not, the multiple instances of indicating “fanatical” would be a tautology.
(There is another use of “tautology” in formal logic that indicates a statement that must be true in every possible interpretation. However, that’s not the use of the word that I wanted to highlight most in this post.)