The past progressive or past continuous is a form of past tense. Different languages use this pas tense in different ways.
Spanish
editIn Spanish it is called pretérito imperfecto and it is used to describe actions started in the past without specifying if the action has been completed or not (e.g. Juan entraba). In contrast, in simple past, the action has been completed (e.g. Juan entró).
English
editVisit Uses of English verb forms#Past progressive/continuous