What are noisy samples in Scikit's DBSCAN clustering algorithm?

If I apply Scikit's DBSCAN (http://scikit-learn.org/stable/modules/generated/sklearn.cluster.DBSCAN.html) on a similarity matrix, I get a series of labels back. Some of these labels are -1. The documentation calls them noisy samples.

What are these? Do they all belong to a single cluster, or do they each belong to their own cluster since they're noisy?

Thank you