Silhouette Plot shows for each cluster:
The number of elements (njnj) per cluster. Each horizontal line corresponds to an element. The length of the lines corresponds to silhouette width (SiSi), which is the means similarity of each element to its own cluster minus the mean similarity to the next most similar cluster
The average silhouette width
Observations with a large SiSi (almost 1) are very well clustered, a small SiSi (around 0) means that the observation lies between two clusters, and observations with a negative SiSi are probably placed in the wrong cluster.