Signal words tell you what tense you have to use. In the Past Progressive we often use a period of time. A typical signal word is while. The Past Progressive is frequently used in sentences together with the Simple Past.
while (In this part of the sentence we usually use Past Progressive.)
While we watch TV, Andy surf the internet.
when (In this part of the sentence we usually use Simple Past. The Past Progressive is used in the other part of the sentence.)
The ladies talk when the accident happened.