what do the "robbers" think ransoming is and what are some objectives to it ?

You're talking about Tom and Huck here? What they believe ransoming is?

According to the text;

“Oh, that’s all very fine to SAY, Tom Sawyer, but how in the nation are these fellows going to be ransomed if we don’t know how to do it to them? —that’s the thing I want to get at. Now, what do you reckon it is?” “Easier said than done, Tom Sawyer. What I’m trying to say is how in the world are the people we kidnap going to be ransomed if we don’t even know how to ransom them? What do you think it means?”

“Well, I don’t know. But per’aps if we keep them till they’re ransomed, it means that we keep them till they’re dead.”

They believe you keep them til their dead.