A reference to someone who appears unexpectedly while being talked about.
This phrase is used to acknowledge the coincidence of someone arriving at a scene just at the time that they are being talked about. Clearly, nothing sinister is implied by this and it is just a jokey way of referring to the person's appearance. In fact, many people using the phrase might not be aware that, prior to the 20th century, the term wasn't meant lightheartedly at all. The full form goes like this - "speak of the Devil and he will appear". The phrase originated in England, where it was, and still is, more often given as 'talk of the Devil'.