I was confused

It means that he was one side of the garden wall and his neighbor was on the other; since their voices cannot carry through the wall, under the wall or around the wall, they have to direct their voices over the wall.

Also, their heads were probably higher than the top of the wall.

he set up to cut the grass

do you have "lawn mower" in china, I'm sure you do.
Many western homes have grass in the front or back of their house. It must be cut reguarily or it will be untidy.



some people have big lawns and use a ride on mower