The security context token

Question

Using SOAP client (SoapUI 5.6.0) we are trying to make a simple call to the ERP system to pull some data, for example, purchase invoice in our case. We imported WSDL and opened a generated request, trying to make a call with default values (basically all the filters are absent so we expect to get all entries), but getting a message: BadContextToken. The message could not be processed. This is most likely because the action 'http://schemas.microsoft.com/dynamics/gp/2010/01/IPurchaseOrderTransactions/GetPurchaseInvoiceList' is incorrect or because the message contains an invalid or expired security context token or because there is a mismatch between bindings. The security context token would be invalid if the service aborted the channel due to inactivity. To prevent the service from aborting idle sessions prematurely increase the Receive timeout on the service endpoint's binding.
What is really a security context and how to use it properly? do you have an example of some basic request to get any entity that includes this security context?