Past Progressive tense is used to express a continuing action that took place in the past.
I was eating my ice-cream.
We were standing near the curb.
The band was playing onstage.
Past progressive tense is also used to express an action that was taking place when another past action occurred. In this case, the second action is expressed in the simple past tense. The subordination conjunction as, when, or while is used to connect the past progressive clause and the simple past clause.
As I was eating my ice-cream, the lights went off.
We were standing near the curb when a yellow Porsche passed by.
While the band was playing onstage, everyone stood at attention.
It is formed by using the past tense of the verb be (was or were) as a helping verb with the present participle.
–based from the book Challenge of the Millenium