An Include connection indicates that the source element includes the functionality of the target element. Include connections are used in Use Case models to reflect that one Use Case includes the behavior of another. Use an Include relationship to avoid having the same subset of behavior in many Use Cases; this is similar to delegation used in Class models.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p.591) states:
Include is a DirectedRelationship between two Use Cases, implying that the behavior of the included Use Case is inserted into the behavior of the including Use Case. It is also a kind of NamedElement so that it can have a name in the context of its owning Use Case. The including Use Case may only depend on the result (value) of the included Use Case. This value is obtained as a result of the execution of the included Use Case.