In PCB design, design rules govern certain behaviour of our circuits. There are industry standard design rules we should aim to follow, especially in specific application areas, like high voltage or high temperature design.
Electrical design rules set clearances between objects, alert for short circuits or unrouted nets, and enforce a clearance distance between components (especially in high voltage devices). In Altium, a green X denotes an electrical design rule violation.
Routing rules determine how traces are routed and how they can be routed. They can also determine routing style and trace width.
SMT rules specify design rules on pads used to mount surface-mounted components. Mask rules define the opening in the solder around a pad or hole, determining how much solder should be applied during assembly.