Tarski's undefinability theorem states that the truth of statements in formal languages cannot be defined within those same languages. This means that while we can express statements about truth, we cannot create a consistent definition of truth that applies to all statements in the language itself. The theorem highlights a limitation in formal systems, showing that any attempt to define truth will lead to contradictions. This result is significant in the fields of logic and philosophy, as it challenges our understanding of truth and the foundations of mathematical reasoning.