As Jeffe already pointed out, the backlist is basically all of the books an author has that are, well, frankly, older. They are not front and center. They are not about to be released, and therefore they end up on the back burner as it were.
naa0w, the trick here, is to do something with those pieces. In the past, well, frankly you hoped someone would come along who could put them out for you.
The past is gone. Celebrate the new norm. These days you can put them out yourself. Something I need to do with several of my titles, frankly. desperately. I have plans, of course, b8ut between life's general chaos and a year and a half of cancer treatments and recovery, I need to get my butt in gear.
I recently, like right before I started dealing with the cancer issues, had a book come out called BOOMTOWN.
I started the novel when my first wife was dealing with hemodialysis three times a week. I put the novel on hold when my wife passed away. It was simply too much to deal with. I couldn't look at the manuscript without thinking about all of the suffering and pain both she and I dealt with as she was dying by inches. As I have been known to express previously, it is what it is.
So I shelved the book. Then, because it was a project near and dear to me, I started writing the sequels to a boo that was not in print. Mostly it was short stories. They came to my mind and I write them for various requested projects. I had not fin9shed the tale of Jonathan Crowley, my immortal monster hunter, in the old west, but I wrote the sequels because I wanted to continue his untold tale. And I gave him a sidekick in the form of Lucas Slate, a cursed individual who had already had a life that was far from normal.
And then, one day, when my mind decided it was time, I picked up that BOOMTOWN manuscript and started writing again. By the time it was fin9shed and sold, I decided it was time to do something with the short stories I had written and collected them in a new book, WHERE THE SUN GOES TO DIE.
They're fun books and I had a blast with them.