Which of the sentences below is correct?
1.We migrated to Australia before couple of days.
2. We had migrated to Australia before coupe of days.
Firstly ‘before couple of days’ is incorrect. I assume you want to say:
A couple of days before
A couple of days earlier
i.e. the migration occurred two days earlier.
I hope that is what you intended.
Your first example uses the past simple. An action completed in the past and not continuing in the present. By saying ‘two days earlier’ we are fixing this event in time relative to ‘now’.
Your second example uses the past perfect. It could equally be used to relate the event to a point in time, but that point of time would also be in the past. For example:
We had migrated to Australia two days before the earthquake. -- both the migration and the earthquake are events in the past.
I had already eaten my dinner when he called.
HTH