Nesting is when you have a complete return flight within an existing trip, but as a separate itinerary.
Example:
In LHR you buy a LHR-LIS-LHR, but you really want to do this trip twice. Say it costs £300 from LHR.
Once you get to LIS, you buy a LIS-LHR-LIS ticket where the return to LIS is before the original ticket's flight to LHR. In other words, your outbound on this flight is your first return to LHR. Say this ticket costs you £200.
Finally, you take your final flight to LHR (from your original ticket). Presto, you have two LHR-LIS return flights for £500 rather than £600.