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It means the same thing but is used differently.
I had been walking away when he called out to me.
I was walking away when he called out to me.
Had been walking away means I was already on route maybe half way across the room.
Was walking way means I probably just started by turning away and took a couple of steps from him.
In most cases, we really don't see a difference in use. The only time this difference is really important is in perfect translation of something technical. You can mostly think of it the same.
I had been watching tv
I was watching tv
There is not a big difference here either. The exact time line of events around this event are the only way to tell which sentence you should use just the same as your example.
ありがとうございます！
理解することが出来ました！