In an analogy, two or more sets of words that have the same relationship are compared. In the case of the analogy lava tube: cave:: jazz: music, the relationship is item/category. This can be explained because a lava tube is a type of cave formed near volcanoes, and therefore cave is the broad category, which includes different items, and one of these is lava cave. Similarly, music is a broad category and within it, there are different items such as rock, pop, and jazz.
