We use the Past Continuous to talk about past actions in progress. The actions can also be interrupted by something (I was eating my breakfast when the telephone rang).
The form of this tense:
Subject + To be (in the past) + Infinitive + Ing
I was reading the book when you come.
When the phone rang, we were eating dinner.
Yesterday at this time, I was still driving the car.
She was always coming to class late (habit)