When the brothers found out that Yosef was alive and ruling over Egypt, they didn't know how to tell Yaakov. They were afraid that the shock might be too much for him and could possibly kill him. According to Chazal, the brothers decided that since Serach, Asher's daughter, could play the harp beautifully, she would be the one to break the news to Yaakov, by singing a song that would include the information that Yosef was still alive. This idea is mentioned in part in the Targum Yonasan Ben Uziel (לבראשית מ"ו:י"ז) where it is used to explain why the Posuk (שם) mentions Serach when no other women are mentioned.
Serach went to Yaakov's tent and began singing soft music to comfort Yaakov, who always continued to mourn the disappearance of Yosef. After Yaakov was settled, the brothers were then able to share with him the details of the news about Yosef still being alive. Despite all this, however, Yaakov at first did not believe what they said, as the Posuk (שם מ"ה:כ"ו) indicates. But when he saw the wagons which Yosef sent, he did believe them (שם פסוק כ"ז). As Rashi (שם) explains, the wagons, which are called עגלות in Hebrew, reminded him of the עגלה ערופה (Eglah Arufah), the subject of the last Halacha which Yaakov had studied with Yosef. Yaakov thus understood that Yosef was sending a message referring to the last Halacha they had learned together, which is information only Yosef could know, and thus his heart filled with joy. Rashi (שם בד"ה ותחי) says that Yaakov felt his Ruach HaKodesh, which was gone during the twenty-two years of mourning for Yosef, come back. Yaakov himself thus called out "Yosef is still alive" (שם פסוק כ"ח). Actually, Yaakov really meant to say that Yosef is still a Tzaddik, and that even during all these years in Egypt, Yosef never lost his belief and trust in Hashem. The fact that he still remembered the Torah they had studied so long ago convinced Yaakov that Yosef was still righteous. We can learn from this that a Jewish parent should always teach Torah to his child because that Torah will remain with him forever.