How does dialect create meaning for the reader?

You have the right idea. Dialect exhibits a person's background and educational level. Listen to hip hop language, or any street lingo and compare it to people who were raised in wealth and educated in exclusive schools. Someone who says "Hey, Ya'all" for (Hello, everybody) comes from the South. Someone who says "Hi Youse guys" for (Hello, everybody) comes from the Northeast, specifically New York/New Jersey. How about "Yo, Sup?" You can guess where that person comes from and probably even his race and age depending on the backdrop of the story. So dialiect has meaning for the reader because he can tell a lot by the way a person speaks.