The line you quote is from Chapter 17 of the book and is spoken by one of the brothers of Buck Grangerford. Its significance is that it foreshadows what will happen in the next chapter.
Buck will get involved directly in the feud, just like he seemed so eager to do when Huck was mistaken for one of the Shepherdsons. He will shoot at Harney Shepherdson but, more importantly, he will end up getting killed by the Shepherdsons towards the end of Chapter 18.
Buck is very eager to get into the fight, but his eagerness will get him killed. The quote you cite is foreshadowing that outcome.