I imagine that an in-process web container would be something like standalone Tomcat where all requests are handled via the same process. An out of process web container would be something like Tomcat running inside Apache where effectively requests are being handled via a different process to that which received them (apache and tomcat running in separate processes).
This may not be correct, but its an extrapolation based upon my knowledge of in process and out of process COM servers where this is the sort of thing that happens.
In In-Process Web container , the main web server and servlet container are different programs but the container runs within the address space of the main server as a plug-in.
in the out-of-process web container, like main server and servlet container are different programs. But the main webserver runs in one process and servlet container runs in a separate process. The webserver uses a plug-in to communicate with the servlet container.