Hey there! We use this verb tense to describe an action that happened in the past at some specific moment. Ex: I WAS sleeping when you called me. We WERE discussing about differences in our culture. Notice that AM/IS turn into WAS; an ARE turns into WERE. I am blonde. --> I was brunette.

You need to study this topic comprehensively. Even after you have learnt the principles, it takes time and practice to feel the difference in usage between this tense and the past simple.