The PCB Editor is a rules-driven environment, meaning that as you perform actions that change the design, such as placing tracks, moving components, or auto-routing the board, the software monitors each action and checks to see if the design still complies with the design rules. If it does not, then the error is immediately highlighted as a violation. Setting up the design rules before you start working on the board allows you to remain focused on the task of designing, confident in the knowledge that any design errors will immediately be flagged for your attention.
Design rules are configured in the PCB Rules and Constraints Editor dialog (Home Tab → Design Rules | Design Rules). The rules fall into 6 categories, which can then be further divided into design rule types. The rules cover electrical, routing, mask, plane, manufacturing and placement requirements.