O que significa "to pour oil on troubled waters"?

When I was in college, some of my classmates and I were invited to a meeting where people gathered together at least once a month to discuss politics. The person that was talking about that meeting made a point to warn us about something before though. She said we should be prepared once we agreed on joining the meeting because it was not unusual for people to go a little rough and throw at each other's throats. But before we got too scared and turned down her invitation, she said there was always someone in charge of pouring oil on troubled water. But wait, do you know what that expression means?

According to a great book called Why do We Say That?, by Jack Scholes, that expression means to do or say something in order to make people stop arguing and become calmer. So imagine you're in the middle of a discussion where people are far from being diplomatic. You're really scared they might get physical, so before that happens you pour oil on troubled waters by changing the sensitive topic to anything else less controversial. That's actually one of many ways of switching off angry people. That's it for now guys. Take care!