Sameness is never specifically defined, but it is used to describe many different aspects of the community in The Giver. It is exactly what it sounds like: everything is the same. The novel never states this explicitly, but it is implied that the ancestors of the current society changed everything at once so that it became the same. Here are some examples of sameness:
There are no hills; the land is completely flat.
The citizens don't see in color, only in black and white.
There is no weather.
Many animals do not exist, like hippos and bears. The citizens believe they are imaginary.
Children hit milestones—like bicycle riding, schoolwork, and physical development—at the same time.
Sameness is not worldwide, though. It exists in the surrounding communities, at least, but it is unclear how far it goes. After Jonas escapes, he bicycles far from the community. The farther away he gets, the less sameness he experiences. He pedals over hills and through snow.
Jonas objects to sameness in part because it limits his ability to make choices. He and the Giver talk about how choice doesn't matter with small things, like tunic color or toys, but it does with bigger things, like spouses and jobs. They agree that sameness is safer, but that answer doesn't satisfy either of them.