Meaning in Context – Is This Expression Correct?

It's correct, or at least very common, in certain dialects of English, but it is not correct in Standard American English (i.e. the formal or semi-formal dialect that's used by newspapers, television news, and the like). In other words, don't use this in formal contexts and don't write this in a paper. If you're learning English, you shouldn't be learning to speak in a non-standard dialect, so you probably don't ever want to use this construction.

As for the people who do use it (just for your information), there is likely little difference between "Do you know where the Walmart is?" and "Do you know where the Walmart is at?"

Here's the inimitable (though that doesn't stop people from trying...) David Foster Wallace on the subject:

This rev. happens to have two native English dialects--the SWE [Standard Written English] of my hypereducated parents and the hard-earned Rural Midwestern of most of my peers. When I'm talking to R.M.'s, I usually use, for example, the construction "Where's it at?" instead of "Where is it?"...

For a dogmatic Prescriptivist, "Where's it at?" is double-damned as a sentence that not only ends with a preposition but whose final preposition forms a redundancy with where that's similar to the redundancy in "the reason is because" (which latter usage I'll admit makes me dig my nails into my palms). Rejoinder: First off, the avoid-terminal-prepositions rule is the invention of one Fr. R. Lowth, an eighteenth-century British preacher and indurate pedant who did things like spend scores of pages arguing for hath over the trendy and degenerate has. The a.-t.-p. rule is antiquated and stupid and only the most ayatolloid SNOOT takes it seriously. Garner himself calls the rule "stuffy" and lists all kinds of useful constructions like "the man you were listening to" that we'd have to discard or distort if we really enforced it.

Plus the apparent redundancy of "Where's it at?"(31) is offset by its metrical logic. What the at really does is license the contraction of is after the interrogative adverb. You can't say "Where's it?" So the choice is between "Where is it?" and "Where's it at?", and the latter, a strong anapest, is prettier and trips off the tongue better than "Where is it?", whose meter is either a clunky monosyllabic-foot + trochee or it's nothing at all.
https://harpers.org/wp-content/uploads/HarpersMagazine-2001-04-0070913.pdf