The 'Online Results' stream shows the online results of full searches. Online results are results from anything but social networks (e.g. websites, blogs, and forums).
More information about creating and maintaining full searches can be found here.
The full search profile will attach a relevancy label to your online search results. Articles will become more relevant depending on the number of occurrences of the words you have put in the search profile. Search terms will be highlighted in a bright yellow color.
The number next to the 'Online results' is the total number of messages 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.