What is the "time of Jacob's trouble"? Who is Jacob today?

Jeremiah 30:5-7 KJV For thus saith the LORD; We have heard a voice of trembling, of fear, and not of peace. (6) Ask ye now, and see whether a man doth travail with child? wherefore do I see every man with his hands on his loins, as a woman in travail, and all faces are turned into paleness? (7) Alas! for that day is great, so that none is like it: it is even the time of Jacob's trouble; but he shall be saved out of it.