Role Binding is the mapping between a Collaboration Occurrence'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 UML specification (UML Superstructure Specification, v2.1.1, p. 174) states:
A mapping between features of the collaboration type and features of the classifier or operation. This mapping indicates which connectable element of the classifier or operation plays which role(s) in the collaboration. A connectable element may be bound to multiple roles in the same collaboration occurrence (that is, it may play multiple roles).