French Imperfective Aspect
The imperfective aspect indicates an action, event or state that was, is, or will be incomplete, in the past, present or future. It describes the process of the situation, its internal temporal structure.
If you say Je lisais ce livre (= I was reading that book), you are describing that process and are probably going to follow up with what happened while you were reading it (as opposed to what happened after you read it).
For example
Note that the imperfective aspect combines the progressive aspect (We were watching) and the habitual aspect (We used to watch).
In French, in the past, the imperfective aspect is usually indicated by the imperfect tense. In contrast, English might use a progressive form, a simple past form, or some other verbal construction.