The short title of this novel is Tom Jones, which is also the name of our hero. This title-equals-main-character's-name equation was a big thing back when Fielding was writing. You can barely throw a stone at a bookshelf full of eighteenth-century novels (if you're into that kind of thing) without hitting one titled after its main character, from Daniel Defoe's Robinson Crusoe to Samuel Richardson's Clarissa to Tobias Smollett's Roderick Random.
So by using his main character's name as the title, Henry Fielding was following the example of a lot of really successful books that came before Tom Jones.
But the formal, long title of this book is actually The History of Tom Jones, a Foundling. And those two words—"history" and "foundling"—are worth thinking about in more detail.
First, Fielding specifically considers the word "history" in the first chapters of both Book 8 and Book 9. He admits that Tom Jones might not technically be a history, since the characters are not real people and there is no proof that any of the events in this book actually happened. At the same time, readers can believe that Tom Jones happened. (At least, this is what Fielding says—we're not so sure that we totally buy every twist and turn of this extremely strange, surprising plot.)
Unlike other "foolish novels" (9.1.1), Tom Jones focuses on consistent, realistic character development and avoids supernatural mumbo-jumbo. Fielding says that the credibility of his novel is what makes Tom Jones more like a history than a "monstrous [romance]" (9.1.1).
Then, there is the word "foundling." A foundling is an abandoned child with unknown parents. Obviously, Tom Jones is both born outside of marriage and doesn't really know his origins, so he certainly fits the dictionary definition of a foundling. But why does it matter to the themes of the novel that Tom is a kid of mystery?
We think it matters because the England that Fielding depicts in this book is filled with two kinds of people: those who care about money (the innkeepers, lawyers, and doctors of the world) and those who care about birth (gentlemen, squires, lords, ladies, and all the servants who depend on them).
And an individual's money and rank depend partly on his parents' places in the social hierarchy. Since Tom doesn't know his folks (at least, until the end), he doesn't have an official standing in the society that surrounds him. He has to prove himself as an individual before Squire Allworthy accepts him back into the family. So Tom is one of the first self-made men of English literature.
What's more, Tom's outsider status means that he can also talk pretty easily to both ordinary common folk and to aristocratic ladies. Tom's hazy background gives him the flexibility to move fairly freely throughout the world that Fielding is portraying. This ability to talk to everybody (because he doesn't really belong anywhere) makes Tom a great central character in a novel that tries to show the extremes of both lower class and upper class English culture.