。
That's just something you thought you heard.
Literal
That [topic-は] you [gen-の] auditory-illusion [copula-だ].
空耳 (literally 'empty ears') is a lovely native Japanese coinage for 'mishearing / auditory hallucination' — the experience of thinking you heard a word or sound that wasn't actually said. It's also the basis of the popular 空耳アワー TV segment where foreign-language lyrics are mistranscribed as Japanese. Very short and purely an opportunity to teach one great word.