To express was.
When talking about action in the past, use the imperfect indicative tense of estar –> estava.
Eu estava almoçando quando você ligou. (I was having lunch when you called.)
Ela estava ocupada ontem. (She was busy yesterday.)
*Don’t use this to express was for talking about permanent things in the past (I was a young boy; She was a sick baby etc.) – use ser for that.