The Send element is used to depict the action of sending a signal, in an Activity diagram. It is the opposite of a Receive element.
The following example shows an order being processed, where a signal is sent to fill the processed order and, upon creation of the resulting invoice, a notification is sent to the customer.
See UML Superstructure Specification, v2.1.1, figure 12.132, p. 408.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 285) states:
SendObjectAction is an action that transmits an object to the target object, where it may invoke behavior such as the firing of state machine transitions or the execution of an activity. The value of the object is available to the execution of invoked behaviors. The requestor continues execution immediately. Any reply message is ignored and is not transmitted to the requestor.