A simplistic model of a robot is to look at as a collection of links connected by joints. The tip of the robot is commonly referred to as the tool center point or TCP. As the joints rotate and the links contract and expand, the TCP will change position.
It is of great importance to know the position of the TCP in world coordinates. For example, for a robot to weld in a straight line, the actuators in the joints of the robot have to be controlled in complex manner.
To facilitate calculations, engineers use the Denavit-Hartenberg convention to help them describe the positions of links and joints unambiguously. Every link gets its own coordinate system. There are a few rules to consider in choosing the coordinate system:
Denavit-Hartenberg convention
Every link/joint pair can be described as a coordinate transformation from the previous coordinate system to the next coordinate system.