Groundswell refers to social trends where people use technologies to get what they need from each other rather than corporations. In an economic crisis, it is more difficult to convince consumers and word of mouth becomes essential. The document discusses how companies can participate in the groundswell by assessing their customers' social activities, deciding objectives, planning strategies, and using appropriate social technologies. It provides examples of how companies can take on roles like listening, talking, energizing customers, and supporting customers through social media.