Progressive Aspect Definition

The progressive, or continuous, aspect is formed with the auxiliary verb 'to be' + - ing, the present participle. It shows that an action or state, past, present, or future, was, is or will be unfinished at the time referred to:

I'm reading Nelson Mandela's autobiography. (action unfinished now)

She was having a shower when the phone rang. (action unfinished at the time the phone rang)