They are only of limited relevance today, but as the technologies involved become more widespread and implemented into every facet of life they will only become more prevalent. While it sounds like the stuff of science fiction, these threats exist now and are not going to go away.
For simplicity, we’ll say that a ‘smart’ device is anything which connects to the internet (or a network) and is not intended to be a computer interface. Intended is the key word there, as many of these devices are insecure for the simple reason that they are a computer. The problem is that it is now cheaper and easier to put a general purpose computer into a device and run some software to, for example, turn lights on and off than it is to design a single-purpose lightbulb which also connects to a network.