What the man in the red sweater teaches Buck is that Buck has to give in to anyone who has greater power than he does.
When Buck first meets this man, he thinks he can just use his power and strength to overcome him. But the man has the club and he knows how to use it. He beats Buck until Buck finally submits.
From this, Buck learns that he has no choice but to submit to a power that is much greater than his own. He realizes that, if he does not, he will die.
The man in the red sweater teaches Buck his first lesson regarding the "primitive law," which will be central to Buck's survival throughout the entire novel.
It was his introduction to the reign of primitive law, and he met the introduction halfway.
The encounter with the man in the red sweater occurs toward the end of chapter one. Buck has been "dog napped," crated, sold, resold, and shipped off to an unknown location. He is mad. Really mad. The man with the red sweater is somebody to take out his anger and aggression on. As soon as the man opens the crate, Buck charges him. Unfortunately for Buck, the man in the red sweater is ready and waiting with a club. Time and time again Buck charges only to be beaten down over and over again.
A dozen times he charged, and as often the club broke the charge and smashed him down.
The man in the red sweater taught Buck about the primitive law. The law is simply this: might makes right. Whoever has the power, must be submitted to.