The Organic positions heatmap in Site Explorer shows the number of keywords a target website or URL ranks for in every position of organic search. The more keywords your site ranks for at a given position, the more orange it is.
When you hover over any portion of the Organic positions heatmap, you can see how many keywords a target URL has ranking (in whichever position) at any point of time.
Note: You can't currently export the information from Position Heatmap.
Above the Organic positions heatmap you can also find handy charts for Organic Traffic and Organic Keywords with an option of tracking historic changes for All countries or any specific one, as well as choosing keywords positions to analyze: