It indicates that everything eventually comes to an end. It applies to happy experiences; one always yearns for the joy to be eternal, but if we are realistic we accept that one day there will be a sof pasuk, “an end to the verse”.
It applies too to the unpleasant times in life – illness, depression, danger, deprivation, misfortune, whatever. One day they will be over. That’s the greatest source of comfort.
It’s not fair that good people have to suffer, and while the pain lasts it can be unbearable, but the suffering will not be eternal.
The book of Psalms is full of this thought. The Psalmist’s constant question is “when?” He never loses faith that God will intervene and turn the darkness into light, but he yearns to know the Divine timetable.
The Chassidic master Rabbi Levi Yitzchak of Berditchev added a further aspect. Addressing himself to God as he constantly did, Rabbi Levi Yitzchak used to say, “Master of the Universe! It is not why I suffer that I wish to know, but only whether I am suffering for Your sake.”
Centuries before, Rabbi Alexandri said, “There is no human being without suffering. Happy is he whose suffering is for the Torah!” (Midrash B’reshit Rabbah 92:1).
One can imagine both Rabbi Alexandri and Rabbi Levi Yitzchak saying, “I can cope without knowing the Almighty’s timetable as long as I know my suffering is for God’s sake”.