Backstory. No-one except the author is really interested in your character's backstory. The reader wants to see what is happening now. Speak for yourself, dear "reader"! Whatever backstory is really necessary can be woven into the main story. Fuck you, damn tunnel visionary. This type of fundamentalistic rules get bent from wire to cater to the nonexisting taste of hoi polloi.
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Flashback. It's just a different form of backstory.
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Prologue. The fuzzy bit at the beginning that doesn’t make sense until you’ve read the whole novel. It's backstory in disguise. Prologues that start a thousand years in the past will cause the author to burn in hell. Okay, you most likely also speed forward over the Paw Patrol theme song.

Chapter one. What? Where else would you start? According to every publisher and agent I’ve met, most novels really start on chapter three or four. The first few chapters are all set-up or backstory which would improve the novel by being deleted. This kinda guys fast forward over porn film beginnings to the first blow job or insertion. Best improvement would be to scrap the whole book. Plus its author.

There are fashions in writing, just as there are in clothes. The modern trend, particularly for genre or YA fiction, but increasingly in literary fiction too, is to start the story with the main character on the first page, and to start with the inciting incident. No backstory before chapter three, and then pare it to the bone. "YA" most likely stands for young adult. There hardly is a brand of monkeys that are stupider than young adults. Except YA writers. Give them a dildo and it will keep them occupied for hours.