Range image segmentation is when a computer program takes a picture of an object and tries to separate different parts of the picture. Imagine you have a picture of a dog, but the dog is on a checkered floor. The computer program could separate the picture into two parts: the dog and the floor.
This is a little bit like when you do a puzzle. When you put the pieces together, you try to separate the different parts of the picture. The computer program does the same thing with the picture it takes.
Range image segmentation does this separation by looking at the different distances between the objects in the picture. This is called the "range." For example, if the dog is closer to the camera than the floor, the computer program can tell the difference between the two parts of the picture based on the range.
In short, range image segmentation is when a computer program separates different parts of a picture based on how close or far away they are, kind of like a puzzle.