Jem realizes that he has been taken in by the oldest trick in the lawyer’s book when Atticus asked him a question as if he really knew the answer when he didn’t, and Jem incriminated himself.
Jem, Scout, and Dill are playing Boo Radley’s story outside where the whole neighborhood can see, and Atticus has asked them to stop because he considers it impolite.
When Atticus confronts them, Jem accidentally tells him what they were doing.
"We weren't makin' fun of him, we weren't laughin' at him," said Jem, "we were just-"
When he asks how Atticus knew what he was doing, Atticus tells him that Jem himself told him. It then took Jem awhile to realize he had been “done in by the oldest lawyer's trick on record” (ch 5). He calls out to Atticus:
"I thought I wanted to be a lawyer but I ain't so sure now!" (ch 5)
The reason this line is significant is because it demonstrates Atticus and Jem’s relationship and how it is developing. Jem is growing up. Atticus trains his children a bit differently than most parents. Since he is a lawyer, he approaches the world from a lawyer’s standpoint.