use it

It is a past tense:
you say that it is finished, over.
It is also a continuous form:
you say that it was busy, happening.
And this tense combines the two ideas:
OVER + BUSY

click here to see the scheme
with the basic idea of the tenses

We often use the past continuous to talk about a background action:
something that was happening/busy at a certain moment in the past.

I was riding my bike when I fell.
(riding was busy, when it happened)
When I saw him, he was fishing.
(fishing was busy when she saw him)
Mother was talking on the phone, when the baby woke up.
(talking was busy, when baby woke up)

In many sentences there are clues that tell you:
use the simple past

Words like:

yesterday, last month (week, year...),
two days (a week, some time...) ago,
in 2010

And there may be an extra clue that says:
use the continuous form

Words like:

while, when, at that time...