The tool collects the behavior of the sites I visit, tracking the connection between them and the possible relations in sharing my browsing history. For each site it can give details about to what other sites is potentially giving my history, and from what sites is potentially getting info. In this sense this tool is “Tracking the trackers”, which is the title of the TED talk about the plugin.
What I like about this tool is that it gives an immediate feedback about the extent of the tracking. I strongly suggest using a cookie blocker such as Cookie Monster, that protects effectively against cookie tracking. You could try to visualize the Collusion network graph with and without a cookie blocker (or Firefox Incognito mode) and really see for yourself the difference that it makes.