The children have taken the legend of Boo Radley and created this monster of a scar-faced man who ate raw squirrels and
"....any cats he could catch, that's why his hands were bloodstained" (pg 13)
His teeth were rotting in his mouth and he drooled a lot. They gave this description to Dill when he first arrived. Dill was seven years old and believed everything they told him.
One day, when the children were bored, Dill challenged Jem to go up to the Radley house and just touch it. Dill offers to trade him The Gray Ghost. All Jem had to do was touch the house.
Jem ran up to the house, hit it with the palm of his hand, and then ran out of the yard so quickly that he didn't even look back to see if someone had come out. Dill and Scout were right behind him, not wanting to see Boo Radley either. However, when they looked back at the house, they only saw a tiny movement of a shutter in the window. Other than that, nothing happened. However, Jem had proven himself a daring person that day.
Jem takes a dare to touch the Radley's house. He runs in the gate and smacks it before running out to Scout and Dill.
This is an example of Jem's early physical courage. He is being courageous without real purpose. Later, he learns it is often more difficult to be courageous when standing up for your beliefs instead of simply putting oneself in physical danger.