It's a histogram illustrating the frequency of the Euclidean distances between your samples. The lowest Euclidean distances (closely related samples) are shaded dark blue, whilst the highest Euclidean distances (distantly related samples) are shaded a very pale green.

The peaks in the histogram just indicate where most Euclidean distances lie, ie:

Euclidean distance 0 (frequency of 8 due to 8 samples, each being Euclidean distance of 0 when compared to itself)

Euclidean distance 18-22

Euclidean distance ~80

You can change the labelling of the key to reflect this with key.xlab="Euclidean distance" and key.ylab="Frequency"
Kevin

thanks Kevin for nice explanation.

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