The past perfect.

Past perfect tense describes an action that happened before another action in the
past. In other words it describes the first of two actions that happened in the past.

When I got to the party, my girlfriend had left already.
After I had watched television, I went to bed.
When I got to the cinema I realised I had seen the film before.

We construct the past perfect using had + past participle (eaten, seen, gone, etc)
When we are telling a story about a past event (I went to the cinema) we might
want to describe what happened before this moment in the past (When I arrived
at the cinema they had already run out of popcorn)
Sometimes the same effect is created by only using the past simple when the
meaning is clear.
After I had been shopping I went home
After I went shopping, I went home.

The most common error with the past perfect is when it is used incorrectly instead
of past simple

When I was at school I had studied maths
(Incorrect, the person did not study maths before going to school)

When I was at school, I studied maths.
(Correct, as both past simple tenses refer to the same period)

Unlike the present perfect the past perfect can refer to a specific moment in the past.
She had been to Sydney in 1981 before her trip to Australia in 1992.

Compare the past simple with the past perfect:
Did you see the start of the film?
Yes it started when we got there No, It had already started when we got there