A source object implements or Realizes its destination object. Realize is used in a Use Case, Component or Requirements diagram to express traceability and completeness in the model. A business process or Requirement is realized by one or more Use Cases, which in turn are realized by some Classes, which in turn are realized by a Component, and so on. Mapping Requirements, Classes and such across the design of your system, up through the levels of modeling abstraction, ensures the big picture of your system remembers and reflects all the little pictures and details that constrain and define it.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.0, p. 110) states:
"A Realization signifies that the client set of elements are an implementation of the supplier set, which serves as the specification. The meaning of 'implementation' is not strictly defined, but rather implies a more refined or elaborate form in respect to a certain modeling context. It is possible to specify a mapping between the specification and implementation elements, although it is not necessarily computable."