They're indicating something that will happen in the future. And that "something" will be an ongoing occurrence. For example, "By that time, she will have been working here for two years.
<span>Lillian will have been talking for two hours by the time she finishes her classical music lecture.
Let's assume that the underlined verb is "will have been talking." The underlined verb is in the tense of future perfect progressive. Because of the word will, it gives us the clue that it is future. Because of the line have been talking will give us the clue that it is perfect progressive.
