After seeing to it that his family was safe on the other side of Yabok River, Yaakov returned across the river because he realized that he had forgotten some of his belongings there. Suddenly, he was attacked by someone who looked like a man, but really was an angel (בראשית ל"ב:כ"ה). Chazal say that this was Eisav's angel, whom Hashem had sent to fight against Yaakov. Yaakov defended himself, but the angel continued trying to bring him down, and the two of them fought all night long. When morning approached, the angel saw, according to Chazal, that the Shechinah was standing over Yaakov to protect him, and he knew that he would not be able to overcome him. But he did move part of Yaakov's thigh out of place, so that he could not walk without limping (שם פסוק כ"ו).
The Torah then forbids any Jew, a descendant of Yaakov, to eat the Gid HaNasheh of an animal, which is a part of the thigh, because that is the part of the thigh where Yaakov was injured (שם פסוק ל"ג). Not eating this Gid HaNasheh helps us remember how the angel moved Yaakov's thigh out of place, but could not overcome him. But why is it important for us to remember this event? What it so significant about it? As stated above, Chazal say that the angel who fought with Yaakov was the angel of Eisav. He therefore represented all of the future enemies of the Jewish people, Yaakov's descendants. The message is that the Jews' battle with their enemies will last a long time; these enemies will constantly try to bring the Jews down. But like Eisav's angel, they will eventually see that Hashem Himself is protecting the Jews, and they will realize that they will not be able to win. They will be able to hurt the Jewish people, as Yaakov was hurt, but not conquer them. By not eating the Gid HaNasheh, we thus recall not only what happened to Yaakov in the past, but what will happen to us, his descendants, in the future. We recognize that just as Hashem protected Yaakov, He will protect all of Klal Yisrael as well.