1. We use the past continuous tense to say that something was happening around a particular time in the past.
I was reading a book at 9 p.m. last night. (I started reading a book before 9 p.m. and I finished after 9 p.m.).
What were you doing at 10 p.m.?
2. We often use the past continuous tense and the past simple tense together. The past continuous is used to "set the scene", or provide (longer) background information. The past simple is used to speak about shorter actions that happened in the middle of the longer action, or that interrupted it:
I was sitting in the garden when it suddenly began to rain.