The imperfect tense is also called the past descriptive tense and corresponds to was doing or used to do in English. The imperfect is used to describe a continued or habitual action in the past, or to describe an action that was occuring in the past, while something else happened.  Time, age, weather conditions as well as mental and physical conditions are all expressed in the imperfect rather the past indefinite tense. 

The imperfect in Italian has the same ending for all three verb groups.  It is formed by dropping the -re of the infinitive and adding the following endings:

Avere is regular in the imperfect, but essere, bere, dire and fare are irregular.  The stem of essere becomes er- for the singular endings, and it does not take the v, while the stem for the plural endings is era- and it does take the v.  The stems for bere, dire and fare are derived from the old Latin infinitives, and are beve-, dice-, and face- and they take the regular endings of the imperfect.

Avevo fame.  I was hungry.
Era tardi.  It was late.
Non diceva niente.  He wasn't saying anything.
Aspettavamo in fila.  We were waiting in line.
Prendevo sempre l'autobus.  I always take the bus.