Backstory in writing tells the story of everything that your character has experienced before the story begins. It takes the form of memories, flashbacks or even allusions. It can tell the reader, for instance, what motivates the character, may help the reader empathize with and care about the character’s outcome.
It also describes your story world before reality sets in. In world building, you may develop the perfect small town, much like Mayberry and then something catastrophic happens. Your frontstory should begin in the catastrophy so in the use of backstory, you can describe how it used to look and what caused the devastation.
Since your story is always told in chronological order, the reader will be able follow when you switch to backstroy, though you must always return to the frontstory to make this possible.
I found an excellent article on this subject and I encourage those new to the craft of writing to give it a read.
All my best to you,