The most obvious explanation for the title, Kidnapped, is that the main character, David Balfour, gets kidnapped. (Surprising, we know.) So the title tells it like it is. But beyond that, the title tells us a lot about the kind of novel Kidnapped is. This is an adventure story, with lots of plot developments that keep the reader in suspense about what's going to happen next. By entitling the novel "Kidnapped," author Robert Louis Stevenson is enticing us to wonder who's going to be kidnapped, what's going to happen to him, how he's going to free himself – all questions that will make us more likely to buy the book (or at least download it from Project Gutenberg).
Kidnapped is sometimes (but not always) given an extremely long subtitle:
Being the Memoirs of the Adventures of David Balfour in the Year 1751; How He Was Kidnapped and Cast Away, His Suffering on a Desert Isle; His Journey in the Wild Highlands; His Acquaintance with Alan Breck Stewart and Other Notorious Highland Jacobites; With All That He Suffered At the Hands of His Uncle, Ebenezer Balfour of Shaws, Falsely So Called. Written By Himself and Now Set Forth By Robert Louis Stevenson.
This has two effects. First, it makes the book seem old-timey. Kidnapped was published in 1886, but it is set over a century earlier, during the historical aftermath of the Jacobite uprisings in England, Scotland, and Ireland in 1745 and 1746. (For more on these, see our summary of Chapter 9.) The adventure novels of the eighteenth century had a certain look to them. Check out at Daniel Defoe's Robinson Crusoe, published in 1719, which has the actual title:
The Life and Strange Surprising Adventures of Robinson Crusoe of York, Mariner: Who Lived Eight and Twenty Years All Alone In an Uninhabited Island on the Coast of America, Near the Mouth of the Great River Oroonoque; Having Been Cast on the Shore by Shipwreck, Wherein all the Men Perished But Himself; WITH An Account of How He Was At Last As Strangely Delivered By Pirates. Written By Himself.
Stevenson seems to be imitating this older tradition of eighteenth-century adventure novels, which used thematic elements like shipwrecks, outlaws, survival – all that good stuff. It's like a marketing tool: if you liked Robinson Crusoe, you'll love Kidnapped.
The subtitle also sets up one of the myths surrounding the book itself, that "David Balfour" is an actual relation of Robert Louis Stevenson's, whose birth name was Robert Lewis Balfour Stevenson. Given the amount of historical research Stevenson clearly did in writing Kidnapped, it makes sense that he would try to make the story seem even more true-to-life and plausible by claiming that the book was written by a real person, a relative, and merely "Set Forth" by Robert Louis Stevenson.
This, sadly, is also fiction: there was no real David Balfour, and the stories of Davie's "Suffering on a Desert Isle" and "Acquaintance with [. . .] Notorious Highland Jacobites" are all made up.