Role Binding is the mapping between a Collaboration Use's internal roles and the respective Parts required to implement a specific situation, typically in a Composite Structure diagram. The associated Parts can have properties defined to enable the binding to occur, and the Collaboration to take place.
A Role Binding connector is drawn between a Collaboration and the classifier's fulfilling roles, with the Collaboration's internal binding roles labeled on the classifier end of the connector.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.216) states:
The roleBindings are implemented using Dependencies owned by the CollaborationUse. Each collaborationRole in the Collaboration is bound by a distinct Dependency and is its supplier. The client of the Dependency is a ConnectableElement that relates in some way to the context Classifier: it may be a direct collaborationRole of the context Classifier, or an element reachable by some set of references from the context Classifier. These roleBindings indicate which ConnectableElement from the context Classifier plays which collaborationRole in the Collaboration