Resource Groups (RG) are courses that cannot be scheduled into the same time period. This would be primarily because of a shortage of resources.
Your school has only one gymnasium so only one PhysEd class can be taught in any one period.
You have only one music teacher, so Music 10, Music 11 and Choir must be held at different times.
Setting up an RG is simple. In the first case above you choose a name, GYM; in the second, MUS. Then you indicate all the courses in these RGs.
RG GYM contains all the courses that use the gym, such as PhysEd 10Boys, PhysEd 10Girls, PhysEd 11BG and PhysEd 12BG.
And RG MUS contains Music 10, Music11 and Choir.
NOTE: In this context it is worth noting that if you have multiple sections of a course, each section will be scheduled by WebSched into a different period. Even though this may not be the optimum placing. The remedy for this is explained under the menu item, Compress Sections.