We use the past continuous when the action is in progress or interrupted by another action, that action is in the past simple.
We form the past continuous by adding the past simple of 'to be' and the verb+ing.
For example:
I was walking when I tripped! ---> the action, 'was walking', is interrupted by 'tripped'.
My parents were fighting until I yelled at them.
My sister was playing Warzone when I called her down for dinner.