past simple is a tense for an action (or fact) that has happened in the past and it's finished in the past also, for example "I left" left is the past tense of leave and it means i left and it's finished with no further information........
But in the past perfect we have the fact and the action which was taking place in that timeline, for example "I was washing the dishes" was is the past perfect tense of be but we express the action that we were doing or was happening...