It's used to emphasize that something is not a copy, imitation, or substitute, but the original and best.
Origin:
The origin of the phrase is somewhat uncertain, but it likely comes from "the real McCoy", a nickname for a famous 19th-century boxer named Norman Selby, also known as Kid McCoy.
Selby was a tough and skilled fighter, and people would often say, "He's the real McCoy," to indicate that he was the genuine article and not a pretender.
Example:
"I thought I was getting a rare antique, but it turned out to be a cheap knock-off. This is not the real McCoy!"