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.