Hierarchical clustering groups observations by finding those that are "nearest" each other and so defining clusters. While there are different ways to define "nearest" and different ways to define clusters, the idea is the same. Here we work with the root anatomy data, and it seems like the sample locations (L1 vs L2) are a bit more clustered than the genotypes.
NOTE! The current version of EssentialR uses a modified form of the data, so the genotypes in this data file are now named A:L and the sample locations are L1 and L2 vs 5-8 and 20-28.