1. Abstract WSDL: Data Type definition (XSD), WSDL (input, output, header...) Messages, Port type with one or more definitions of operations. This type of WSDL just gives you information about the operation and its input and output messages. This is incomplete in terms messaging protocol and transport definition.
2. Concrete WSDL: Abstract WSDL + Binding (Messaging protocol) and Port definition (Transport details). This type of WSDL is complete and can be used to consume the service.
Your question relates to WSDL Binding. This makes your operations defined in the port type concrete, meaning: it binds your operation to a messaging protocol, usually SOAP. When doing this, it is necessary to define the soapOperation and it provides the information for port type operation as whole.
<soapperation stype="rpc|document" soapAction="">
The soapAction attribute specifies the SOAPAction header for this operation. The value for it must be specified only in case of HTTP transport. This is usually used for message routing purposes.
I recommend reading SOAP specifications for further details.
Hope this helps
Regards, Sujeeth Pakala
SCJP, SCWCD, SCBCD, ICAD, XML Master Basic, Certified SOA Professional