It means two letters have been transposed. Two letters have been swapped. Two letters have been interchanged.
You get an email that seems to be from a big, legitimate company, like the Bank of America. It asks for personal information that could be used for identity theft. You foolishly think you can trust them. You reply simply by hitting the "reply" button. You don't notice that the reply isn't going to bankofamerica.com, it is going to bankofamercia.com.
The two letters, "ic," have been flipped around to "ci."
The bad guys have bought a domain that is deceptively similar to Bank of America's. Your personal information isn't going to the Bank of America, it's going to whomever bought the domain name "bankofamercia.com."