Descrever uma ação que estava acontecendo numa certa ocasião do passado.
At two o'clock in the morning, (tempo já passado)
the action was still happening (still: ainda; to happen: acontecer).
The people were having a good time. (to have a good time: divertir-se).
Passado do verbo to be (was/were) + -ing forma do verbo principal.
I/He/She/It was playing.
You/We/They were playing.
Fonte: www.brasilescola.com