From the free online book by Ciaran McHale, CORBA Explained Simply :
Operation parameters have a specified direction, which can be in (which means that the parameter is transferred from the client to the server), out (the parameter is transferred from the server back to the client) or inout (the parameter is transmitted in both directions).
Thus, the in parameter is very similar to the "traditional" function parameters in that the caller must pass a value for them, and this value is used by the server to do its job.
The out parameter is similar to the return value, so the caller never fills it with a value. It simply magically matters when the function returns (provided that no exception has been thrown), because the server is responsible for putting the value inside it as part of its execution rules. You can have as many out parameters as you want, allowing you to return several different objects or values without first combining them into a struct .
The inout combines these two concepts. The caller must fill in all inout valid data, but after the function returns, these values may differ, since the server can host new data there.