The children spit on the hinge to keep the gate from squeaking.
The Radley gate is not the most attractive thing in the world. Like most everything at the Radley house, it is in disrepair. It is “hanging crazily on its homemade hinge.”
One night, Dill and Jem decide to “peep” in the Radley window to get a look at Boo. Scout does not want to do this. She asks them why they waited until the night. Their logic is unassailable. It is better to look in at night because they can see more easily, but no one can see them. Also, if they die they will miss school instead of vacation.
With that, I had no option but to join them. We thought it was better to go under the high wire fence at the rear of the Radley lot, we stood less chance of being seen. The fence enclosed a large garden and a narrow wooden outhouse. (Ch. 6)
The children are trying hard not to be seen or heard, so a squeaking gate is very dangerous indeed. It is Dill’s idea to spit on the gate, and all three children spit themselves dry. Jem opens the gate successfully, unfortunately the children are heard and Nathan Radley comes out with a gun looking for a prowler.
They run, and after they make it safely back Atticus and the other adults discover that Jem is missing his pants. He left them at the Radley place when they got caught. This leads to Jem needing to sneak out of the house at night and risk his life to get them back, so Atticus won’t find out what he did and be disappointed in him for not listening.
In chapter 6 of To Kill a Mockingbird, the children silence the squeaky garden gate to the Radley's backyard by spitting on it.
"We came to the gate that divided the garden from the back yard. Jem touched it. The gate squeaked. “Spit on it,” whispered Dill. “You’ve got us in a box, Jem,” I muttered. “We can’t get out of here so easy.”“Sh-h. Spit on it, Scout.”We spat ourselves dry, and Jem opened the gate slowly, lifting it aside and resting iton the fence. We were in the back yard."