Pretty much exactly that. Track down people who are trying to change the timelines, and stop them. You really do have to keep a tight rein on that sort of thing.
[He lifts an eyebrow of his own.] Have you not met the Doctor? Most people here seem to have done, at one point or another. He's been here several times, albeit most of them very short. Never could keep that man still for long.
He's a Time Lord. He basically flits about through time and space saving people... planets... sometimes the whole universe. Martha and I both travelled with him, for a while. [He grins, briefly.] Best few months of my life, in some ways.