It's a verbal tense that is use for actions that were complete in the past, this actions could finish 1 hour ago or 2 years ago.

I ate pizza with my friends 3 weeks ago.

3 weeks ago, I ate pizza with my friends.

This tense is use for describe actions that happen in the past and other actions interrupt tha first one or explain two or more actions that happend in the past.

I was reading a book when my brother broke a window.

While my brother was eating a taco, I was reading a book.

We use this tense to explain actions that we were used to do in the past with frequency.

When I lived in Mexico City I used to wake up early.

We use this tense to express permanent actions, actions that repeats in the time, express actions that repeat every day.

I walk to my home after school.

Lima is the capital of Peru.

We can use this tense for describe actions that are happening at
the moment.

He is studying serbian.

This is use to describe actions that start in the past and continous in the present.

I had lived in Laos since 1998.