I am struggling to get my head around the concept of "unit root" in relation to time series. And it would be a great help if someone could give me a two or three sentence intuitive explanation of the concept.
As an example of the kind of thing I am after, earlier on I also wanted to find the meaning of "hurst exponent"... and it was a great help to find out that it was simply "a measure of the smoothness (as opposed to crinklyness) of the time series" - see image below.
EDIT: Alternatively, a pointer to an online explanation would be good, so long as it includes a good helping of "Tell them what you are going to tell them".