A 'process diagram' is any diagram that shows steps or stages in a process. Many students worry about this type of question, but they are really quite easy if you know what to do.
Look at the following example from Cambridge IELTS 1, page 51:
The diagram below shows how the Australian Bureau of Meteorology collects up-to-the-minute information on the weather in order to produce reliable forecasts.
- Introduction: rewrite the question in a different way
- Summary: say that there are 4 steps, and briefly mention each one
- Main body: describe steps 1 and 2
- Main body: describe steps 3 and 4