Past progressive tense is used to indicate that an action was in progress when another action occurred in the past.
Past progressive (was/were + Ving) refers to the action in progress, and simple past tense refers to the "interrupting" action.
While I was eating dinner, the telephone rang.
The telephone rang while I was eating dinner.
When the telephone rang, I was eating dinner.
I was eating dinner when the telephone rang.
Past continuous can also be used to indicate only one action which occurred in the past:
What was John doing yesterday?
He was working at the factory.
What were they doing yesterday afternoon?
They were playing soccer.
What were you doing last night?
I was studying English.