The 'Social results' stream shows the social results of the full searches and quick searches. This stream used to be more comprehensive, but because a lot of channels are more restricted due to privacy it's only available for Twitter now. The keywords you set in the searches are highlighted in an obvious bright yellow marker color. With filters (the upside down triangle) you can choose to show quick search or full search results.
Everything you should know about the searches can be found in the 'Search profiles' section.
The number next to the Social results is the total number of results in the stream.
The meaning of the three icons in the upper right corner of the stream window is explained here. They allow you to edit the stream and select one or several filters.