These days researchers are asked to assess the impact of their research. It's not good enough to say that you're doing it out of a quest for ever greater knowledge, or because as humans we should strive to find out how our world works. It also isn't going to wash if you point out the great amount of mathematical work that seemed to have no practical application when it was first done but now underpins and is vital for absolutely everything in our day to day life (computers, for instance - the principles behind them preceded the technology by centuries).
So we need to know how to measure impact, or at least how to convince the funding bodies and the REF that we are doing stuff with impact. Even though we don't need to worry about the REF yet, as PhD students, it's wise to get to know about this stuff early.
With that in mind, here's a link to an LSE site with podcasts on just this, aimed at us folk. There are also other resources on the site, and they are holding an event on the 1st December for PhD students.