The DLookup function looks up a value from a specific field in a specific table where the criteria matches what you want eg on a form If you want to populate an address field based on a employee you could for example in the controlsource of the address field enter
=DLookup("[StaffAddress]","[Staff]","[StaffID] =" & Forms!ThisForm!StaffID
In english this reads - Lookup the Staff Address in the Staff Table where the StaffID is equal to the current StaffID on the form.
You can do this easier by adding the field to the underlying form query but you can use this method in several ways.
Explain your data storage a bit more clearly and what you want to automatically fill in.