The simplest form of AI is a rule-based system. This type of system uses a set of rules to determine what action to take in order to achieve a goal. For example, a rule-based system might be used to control a robot arm in a factory. The system would be programmed with a set of rules that define how the arm should move in order to pick up and place objects.
A rule-based system is not as flexible as some other AI systems, but it can be easier to implement and understand. Additionally, rule-based systems can often be extended or modified by adding new rules or changing existing ones.