A basic parsing rule would be one which extracts a data value which can be found inside the body of the email. Ideally this value has a preceding label which stays the same in all emails (e.g. "First Name:", "Order ID:", "Shipping Address:", ...).
- Select a data source (e.g. text body)
- Add a first text-filter which defines the start of your variable (e.g. search for "Order reference: ")
- Add another text-filter which defines the end of your variable (e.g. "end of line")
- Refine the result with additional text filters
The following screencast is a good starting point. It shows you how you can build up your own parsing rules step by step.