To pull the passage in question from the King James Bible:
24 And Jacob was left alone; and there wrestled a man with him until the breaking of the day.
25 And when he saw that he prevailed not against him, he touched the hollow of his thigh; and the hollow of Jacob's thigh was out of joint, as he wrestled with him.
26 And he said, Let me go, for the day breaketh. And he said, I will not let thee go, except thou bless me.
27 And he said unto him, What is thy name? And he said, Jacob.
28 And he said, Thy name shall be called no more Jacob, but Israel: for as a prince hast thou power with God and with men, and hast prevailed.
29 And Jacob asked him, and said, Tell me, I pray thee, thy name. And he said, Wherefore is it that thou dost ask after my name? And he blessed him there.
30 And Jacob called the name of the place Peniel: for I have seen God face to face, and my life is preserved.
31 And as he passed over Peniel the sun rose upon him, and he halted upon his thigh.
32 Therefore the children of Israel eat not of the sinew which shrank, which is upon the hollow of the thigh, unto this day: because he touched the hollow of Jacob's thigh in the sinew that shrank.
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There are a couple of things going on here. Apparently, Jacob is the second of two twins, the first man out the hatch being Esau. Esau is the outdoorsman, the physical one. Jacob is the thoughtful one. So it's unusual for him to find himself in this physical conflict. Yet, he is able to struggle, toil, and grapple all night, and into the dawning of the sun.
The passage says nothing about the "man" being an angel until god reveals himself and knocks Jacob's hip out of joint with his pinky finger. We may take this more to mean this struggle was psychic, or metaphorical - that the man is an externalization of Jacob's belief. In the end, this struggle is vitally important, because Jacob is named Israel by the Lord
On a more quotidian note, we get a good old Hebraic dietary prescription about avoiding hip-meat. It always comes back to food.