If the input for a particular service is to be combined from more than one inputs or upstream services, these inputs can be combined in a special Merge operation. This allows you to feed the merged data in to the next service. That is the Merge operation create a list from all the inputs.
Merge operation is simply a list creator.
Merges appear in the Workflow Diagram as circles.
Taverna creates merges automatically if you create more than one data link to the same service input port or workflow output port.