In Harper Lee's To Kill a Mockingbird, Jem follows the court trial closely. Having grown up in the home of a lawyer, Jem and Scout understand the workings of the courtroom, and they have come also to recognize how Atticus behaves and what his specific behaviors signal.
The quote that I believe you refer to comes after all the testimony has been given. Jem would know not to draw any conclusions until after all the evidence was in, including the testimony of Heck Tate, Bob Ewell, and Mayella Ewell. When this has been done, but before closing arguments, Dill feels unwell and Scout goes outside with him so he can regain his "equilibrium."
Upon their return, Scout demands an update from Jem. When Scout punches Jem to get solid information, he responds:
He's just gone over the evidence...and we're gonna win, Scout. I don't see how we can't. He's been at it 'bout five minutes. He made it as plain and easy as—well, as I'da explained it to you. You could've understood it, even.
Jem's belief that Tom will be acquitted stands in start contrast to what actually occurs. Tom is found guilty and Scout describes how Jem reacts:
I shut my eyes. Judge Taylor was polling the jury: 'Guilty...guilty...guilty...guilty...' I peeked at Jem: his hand were white from gripping the balcony rail, and his shoulders jerked as if each 'guilty' was a separate stab between them.
This signals a growing experience on Jem's part. He was, with a rational mind, sure of Tom's innocence and his father's ability to free Tom. However, he had not counted on the strength of bigotry on the part of most of the jurors, and not only is he devastated, but he sees the world through eyes enlightened by the terrible injustices adults visit upon each other.
Although Scout is never sure about the outcome of the trial, Jem shows supreme confidence in his father's lawyering ability. Jem also seems to see what the jury does not see--that Tom's crippled arm prevented him from the possibility of beating or raping Mayella. Jem spoke words of encouragement to Scout several times during the trial. After Scout returned with Dill following their break with Dolphus Raymond in the courtyard, Atticus was just about to begin his summation. Jem told Scout
"... and we're gonna win Scout. I don't see how we can't."
After the children returned home from eating, Jem "happily" reassured Scout again; he seemed to think the long wait meant a not guilty verdict for Tom. He told Scout
"There are things you don't understand."
Jem and Scout are sitting in the balcony talking with Reverend Sykes while waiting for the verdict in Tom's trial. The Reverend says that he thought Judge Taylor's charge to the jury before sending them out to deliberate was fair, even "leanin' a little to our side." Jem replies to him:
"He's not supposed to lean, Reverend, but don't fret, we've won it," he said wisely. "Don't see how any jury could convict on what we heard--"
When the Reverend tells Jem that he should not be so confident and explains that he has never known a jury to "decide in favor of a colored man over a white man," Scout remembers Jem's response:
But Jem took exception to Reverend Sykes, and we were subjected to a lengthy review of the evidence with Jem's ideas on the law regarding rape: it wasn't rape if she let you, but she had to be eighteen--in Alabama, that is--and Mayella was nineteen. Apparently you had to kick and holler, you had to be overpowered and stomped on, preferably knocked stone cold. If you were under eighteen, you didn't have to go through all this.
Jem, as it turns out, was right on the law and the lack of evidence presented at Tom's trial, but he was naive about the jury's reaching a just verdict.