The PAST PROGRESSIVE TENSE indicates continuing action, something that was happening, going on, at some point in the past. This tense is formed with the helping "to be" verb, in the past tense, plus the present participle of the verb (with an -ing ending):
The past progressive indicates a limited duration of time and is thus a convenient way to indicate that something took place (in the simple past) while something else was happening:
The past progressive can express incomplete action. (as opposed to the simple past, which suggests a completed action: The past progressive is also used to poke fun at or criticize an action that is sporadic but habitual in nature:

Generally, progressive forms occur only with what are called dynamic verbs and not with stative verbs. If you wish to review that concept now, click HERE.