I've been listening this expression from movies and it seems that native speakers used to say "What's that mean" when they want to ask about the meaning of something.
Is this expression right for informal speaking or is it just my misunderstanding?
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I had the same pronunciation question and I found the answer to your question in this Youtube video that explains exactly why it sounds like that.
One of the meanings of 's is "does" - this is often forgotten. We say that 's means "is" or "has" and forget that it can mean "does".
Some answers and comments wrongly regard this usage as incorrect or as dialect.
It is an informal usage but it is neither incorrect not dialectal. Lexico records it here https://www.lexico.com/definition/'s