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. You can also create Send events using the 'Event' icon on the 'State' page of the Diagram Toolbox.
This 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 the OMG Unified Modeling Language specification, (v2.5.1, figure 12.132, p.408.)
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.452) states:
A SendObjectAction is an Invocation action that transmits any kind of object to the object given on its target InputPin. The object to be transmitted is given on the single request InputPin of the SendObjectAction. If the object is a Signal instance, then it may be handled by the target object in the same way as an instance sent from a SendSignalAction or BroadcastSignalAction. Otherwise, the reception of the object can only be handled using an AnyReceiveEvent (...).