Hi,
Most heatmaps have been clustered in order to show the most related data together. There are many different clustering algorithms, but if you use the heatmap() function in R, the default is hclust, a hierarchical clustering. "hierarchical " here is key as it is the basis for the dendrograms.
Dendrograms show how related data point are. Here is a short blog that introduces the topic:
Details on the heatmap function are found in the documentation
Hope this helps,
PJ