The frequency plot divides the display into squares and counts the number of birds that were seen in each square (Figure 1). When viewing "All" in the bird filter (the drop-down on the toolbar), the number of birds seen in each square is displayed. When filtering by bird (only showing American Robin, for example), the number displayed and the shading of the square is determined from the average number of birds seen in each visit to the square.
Figure 1 - The Frequency Plot
The "Consolidate" drop-down on the toolbar controls the size of the square. It can range from "Very Small" (shown in the screen snapshot above) to "Huge", which covers an area approximately equal to a small city (Figure 2).
Figure 2 - Frequency Plot (Wider Scale)