Huck and Jim spend sometime talking about how Jim life is going to be like when he officially becomes a free man. Jim starts talking to Huck about how thanks to him, he is a free man. Huck felt for this because he knows its his fault why Jim ran away and how the police are looking for him. Huck didn't mean to put Jim at blame for his "death." Huck wanted to tell Jim he was sorry, but Jim was thanking Huck for being his one and only friend, and knowing he could trust Huck to be there to help him. After that Jim talked about the things he would do when officially became a free man.
I think that sweet of Jim to tell Huck how much their friendship means to him. You know that your friendship is strong when you can really trust that person to be their for you when your down or when you need someone to talk to. Friends are there to back you up and stand up for you. That's how strong Huck and Jim's friendship is.