Backstory is history that helps your reader to better understand the motivations and inner lives of your characters, it can help them to care about and identify with your characters. Unlike a history lesson, backstory should be brief, giving enough information to enlighten, without disrupting the forward motion of the story.
When deciding where to include backstory, be mindful of placement – make it relevant to the context, and keep the length of exposition proportionate to the pace. If backstory is inserted between lines of dialogue it should be no longer than a line or two, it can be longer if it occurs during a lull in the action.
Help your reader to easily distinguish between present action and backstory by using clear transitions when introducing the backstory and when bringing the reader back to the action. One way to do this is to use an object, such as a vase or a photograph, as the ‘trigger’ or conduit into backstory, and as a point of reference when you return to ‘the present’. This will help your reader to stay oriented.