In the previous aliyah and in this, the fifth aliyah, Yosef acts in a rough way to his brothers. While they're experiencing this they express regret about what they did to Yosef. Yosef accuses them of being spies, and holds Shimon hostage while sending the rest back to Canaan to fetch Binyamin.
The reason Yosef kept Shimon was to separate him from Levi. Together, they had defeated the whole city of Shechem earlier in the Torah. Yosef is trying to prevent that from happening again in Egypt.
When the brothers get back to their father Yakov, he is upset at the thought of another son, Shimon, being missing, and the possibility of even another one, Binyamin, being lost. Therefore Yakov hesitates for a long time to send Binyamin, but in the end, because of the famine, he does send him with them.
