Flashbacks
   
A flashback is one method of inserting backstory.  When authors feel something that happened before the current story began is relevant, they can stop the forward motion of the story and include the past event or information as a flashback. 

A flashback may be as short as a few sentences of narrative or may run for several pages and may contain dialogue, action, setting and description.

Here’s a brief example of a narrative flashback

***She had divorced her husband and her lawyer had been successful in negotiating a large cash settlement.  Soon she had settled into a life of lavish indulgence, which included traveling with and supporting younger men.  I know because I had been one of them.  Before long, she had gone through the money and the men had drifted away.  Now she had been reduced to waiting tables at a diner in the seediest part of town.***

Writers often use past perfect verbs (“had” and “had been,” for instance) in a flashback so readers will know they’re reading something from the past.  This is considered passive writing, however, and can be avoided if a distinct transition is used to introduce the flashback.  A good transition takes the reader into the past without a need for past perfect tense. Here’s the same example with “Ten years ago” as the transition, the passive verb helpers eliminated, and some addition tightening:  

***Ten years ago, she divorced her husband and her lawyer negotiated a large cash settlement.  She then settled into a life of lavish indulgence, which included traveling with and supporting younger men.  I know because I was one of them.  The money and the men are gone now and she waits tables at a diner in the seediest part of town.***

But a clear transition is also needed to bring the reader out of the flashback and back to the present story. 

Here’s one way:

***I remembered all that about her as I settled into a booth and she trudged over to take my order.  I also wondered if she remembered me.

“What’ll you have,” she asked without any sign of recognition.***

While some people object to flashbacks, there are times when they can be used effectively if they are brief and you have a clear transition into and out of them so readers don’t get confused.