The similarity matrix is a simple representation of pair combinations, intended to give you a quick insight into the cards your participants paired together in the same group the most often. The darker the blue where two cards intersect, the more often they were paired together by your participants.
It also clusters related pair combinations together: the strongest pair is grouped next to the second-strongest pair that either of the first cards have, and so on. The algorithm (similar to the best merge method Dendrogram algorithm) attempts to cluster the similar cards down the right edge. Clusters are presented in the same shade of blue.
To explore the Similarity Matrix in more depth, hover over any square to highlight the two cards, and to see the exact number of participants who paired them together. In our example, 60% of participants, or 24 people, put the cards 'Purchase a cell phone online' and 'Accessories for my cell phone' in the same group: