The past progressive is used when referring to a moment in the past, during which something else was happening. In other words, something that started before and ended after a specific time in the past. To form the past progressive, we use a noun or personal pronoun + was/were + verb+ing.
I arrived at the concert around 8 pm. The band was already playing. (The band started playing before, and continued playing after I arrived.)
She called me last night, but I was already sleeping. (I fell asleep before she called, and was asleep after she called.)
We wanted to go to the flea market yesterday, but they were already packing up.