It is the full remission of all temporal punishment owed to our sins up to the point we obtain it.
So if you’d been away from the Church for 20yrs (as an example), made a general confession, met all the requirments for obtaining a plenary indulgence, then all punishments due to all those sins up to that point in time has been remitted.
If you are a practicing Catholic who regularly goes to confession say every month for example, then all the temporal punishment you’ve incurred since the last time you obtained a plenary indulgence has been remitted (less of course any that was previously remitted through partial indulgences in the intervening time).