The Jewish concept of the Messiah is one of the most misunderstood in Judaism. There are several reasons Jews have not been open about discussing this idea with the world until recent decades, when the Lubavitcher Rebbe, Menachem Mendel Schneersohn, directed that Jewish world that it was time to bring the Messiah with “Acts of Goodness and Kindness.” (he said this on CNN, and I suspect that Rabbi Schneersohn was probably the first Chassidic Rebbe to appear on cable TV!) Actually, the Chofetz Chaim, who lived in the middle of the Twentieth Century, also discussed the importance of bringing the Messiah by cutting out unflattering talk about others or Lashon Hara. Still, discussing the concept of the Messiah has been rather new in recent times.
One of the reasons Jews did not share views about the Messiah with the world (awaiting the Messiah is literally all over Jewish liturgy or prayer and scripture, but seldom crept up in ordinary conversation), is that there was a reluctance to seem as if Jews were “borrowing” the concept from other religions, although the idea of the Messiah began with the Jews. Because the Jews were awaiting the Messiah, and others believed in a Messiah who had already arrived, there was a disinclination to get the two views confused.
Basically, the Messiah in Jewish thought is a descendant of King David who will also be a king and will bring the Jews together to live in the Land of Israel and to keep the commandments. He will fight wars against the enemies of the Jews, will prevail, and will inaugurate an era of everlasting peace when the Third Holy Temple will be built in Jerusalem. It is commandment to long for the Messiah every day and to expect him to arrive. There is no time that it is impossible for the Messiah to come, even in the darkest hours.