This means that you can keep what you find unless someone else proves he owns it.
This is a legal reference. “Title” refers to an ownership document, such as the legal paperwork for a car. "Finders keepers" is, I am sure, a phrase you have heard. It means that the person who finds something abandoned gets to keep it, because the person who left it has no right to it anymore.
Finders were keepers unless title was proven. Plucking an occasional camellia, getting a squirt of hot milk from Miss Maudie Atkinson's cow on a summer day, helping ourselves to someone's scuppernongs was part of our ethical culture, but money was different. (Ch. 4)
The implication here is that most things that you find you can keep, because it is harmless. You can pick flowers or take a squirt from a cow, because there is plenty more where that came from. Money, on the other hand, is a rare commodity. You are not supposed to keep it when you find it.
Thus when Scout and Jem find two Indian-head pennies, they are at a loss on what to do.
"Grown folks don't have hidin' places. You reckon we ought to keep 'em, Jem?"
"I don't know what we could do, Scout. Who'd we give 'em back to? (Ch. 4)
The pennies have been left there by Boo Radley, in an overture of friendship. The children do not know that though. They are not sure if they should keep them, because money is very valuable.
In a way, the children are right. Boo Radley is not a grown up. He is more of a child trapped in an adult's body. He is desperate for a friend, and he does not want to be isolated any more. He watched the children, and he saw that they understand him. He decided to reach out to them, by leaving them the pennies.