He knows his son wants to be a lawyer and is interested in law.  He is proud that Jem has noticed how juries are made up.

Atticus leaned back in his rocking-chair. For some reason he looked pleased with Jem. "I was wondering when that'd occur to you," he said. (ch 23)

Atticus explains that women cannot serve on juries, and people in town would all have a conflict of interest because they might be afraid to sit in judgment of someone else who is a neighbor or customer. 

Basically, Atticus is proud because Jem is being more mature.  Scout does not understand why Atticus is pleased.  She does not realize that Atticus sees Jem as a chip off the old block.  He is glad that Jem is smart, but also that he is following in his father’s footsteps and developing a legal mind. 

Jem is very concerned when Tom Robinson is convicted.  It offends his sense of fairness.  This observation shows that Jem is becoming more mature and moving on.

As was mentioned in the previous post, Atticus is proud of Jem's ability to closely observe certain aspects of Tom Robinson's trial in Chapter 23. Following the trial, Jem becomes jaded after witnessing racial injustice firsthand and asks his father numerous questions regarding Maycomb's court system. On one occasion, Jem makes the keen observation that the juries are seldom made up of citizens from Maycomb. When he asks his father why townspeople never serve on juries, Scout mentions that Atticus seems pleased at Jem's observation. Atticus goes on to explain that townspeople are both afraid and uninterested in serving on juries. The citizens of Maycomb do not want to serve on a jury out of fear that their decisions might hurt their business or reputation. Atticus then tells Jem that one of the Cunninghams was actually arguing for an outright acquittal during the deliberation. 

In chapter 23, Atticus is impressed that Jem is able to observe that townspeople do not serve on juries in Maycomb.

Jem is disturbed by the jury verdict in the Tom Robinson case.  He is concerned because the jury convicted a man that Jem considered blatantly innocent. 

Suddenly his eyes widened. "Atticus," he said, "why don't people like us and Miss Maudie ever sit on juries?  You never see anybody from Maycomb on a jury- they all come from out in the woods." (ch 23)

Atticus is impressed, and explains to Jem that the townspeople do not want to offend anyone.  They have to live close together, and many of them own businesses.  None of them are impartial.

Serving on a jury forces a man to make up his mind and declare himself about something. Men don't like to do that. Sometimes it's unpleasant. (ch 23)

It would interrupt town commerce to have Maycomb citizens on juries.  Also, women cannot serve on juries at all.

This observation is evidence of the fact that Jem is getting more mature.  He is old enough to observe and question how the world works.  This is significant because he is experiencing some difficult times that will be foundational in the person he becomes an adult.