Where did you come from? What have you done? Beyond those given by your class, what are your skills? You get to choose a Background for your PC, which can be any single trade or occupation name, which captures this. When you attempt a task that your Background gives you expertise in, you get a bonus (unless you’d get a bonus anyway because of your class).
A mechanically-useful background is one that complements your class, giving you expertise (and thus a bonus on an attribute check) where your class would not.
The game assumes that level 1 characters are net broke, apart from their initial money allocation, and do not have access to significant local resources. So if you define a background that implies such (e.g. “Merchant Adventurer”) then it is implicit that you have fallen on hard times at the point you enter the game.
Examples — scholar, mercenary, blacksmith, prostitute, noble, sculptor, chef, travelling tinker, farmer, galley slave, …