The "Action" is one of the strings in the message header.
For example, this call
var m = Message.CreateMessage(MessageVersion.Default, "http://tempuri.org/MyMethod");
Produces this message
<s:Envelope
xmlns:a="http://www.w3.org/2005/08/addressing"
xmlns:s="http://www.w3.org/2003/05/soap-envelope">
<s:Header>
<a:Action s:mustUnderstand="1">http://tempuri.org/MyMethod</a:Action>
</s:Header> <s:Body />
</s:Envelope>
Each message has an "action" header, and each WCF operation has an "action" attribute. The WCF system will compare these values when determining which operation to dispatch each message to.
Normally, you aren't manually generating messages so you don't have to worry about this - it's all handled as expected by default values.
When you define the Service Contract you can explicitly associate an action string with an operation:
[ServiceContract]
interface MyService
{
[OperationContract(Action="http://tempuri.org/MyMethod")]
void ThisIsntReallyCalledMyMethod(string parameter1);
}