Overcoming the social awkwardness often experienced between people meeting for the first time. One might break the ice by making a funny joke or finding another way to ease the tension.
Example of use: John and David were both a bit uneasy when they first met, but they broke the ice with drinks at the bar before they began discussing business.
While some might connect the idiom to the 18th century when special ice-breaking ships were built to help explore he polar regions, the first recorded use actually goes back to the 17th century (1678) when Samuel Butler used it in his poem Hudibras. "The Oratour - At last broke silence, and the Ice."