Be aware that the Altium documentation goes into much more detail than necessary for our purposes. The best way to determine what you don't know is to start a project.
Getting StartedMore Details: http://techdocs.altium.com/display/ADOH/Getting+Started+with+Altium+Designer
...
Design rules are what defines the constraints (rules) for every aspect of the PCB. These rules are mostly distances between objects and will be discussed in more detail here. The design rules must follow the manufacturer's specifications and tolerances in order for the PCB to be made with as few errors as possible. The Design Rule Checking (DRC) tool is an automated way to check if your PCB has violated any of the design rules, and should be used throughout the PCB layout process, most importantly before gerber creation.
...