Priority rules were the subject of the last eBook in this series. Students who master those are ready to progress to applying those rules to chiral molecules. Not before though, because it is much harder to master two or more tricky concepts at once.
Molecules with just one chiral center comprise enantiomers. Equal amounts of enantiomers are racemic, if one predominates over the other then there is optical activity due to the enantiomeric excess (ee) of one over the other. One pure enantiomer is optically pure (100% ee). The definition of ee seems over complicated. For instance, why describe a 95:5 mixture of enantiomers as having 90% ee). However, defining things that way makes perfect sense when reading optical purities on a polarimeter because rotation of plane polarized light due to the minor enantiomer (5% in this case) cancels 5% of the rotation due to the major one, so 90% of the maximum value is read.
Priority rules are used to assign chiral centers as R or S, and to describe reactions forming them selectively as happening on the Re or Si-faces.
Molecules with more than one chiral center exist in diastereomeric forms. They are called epimers if they differ at only one chiral center. Some compounds have a diastereomer with an internal plane of symmetry. In those cases one or more chiral centers are mirrored by those having opposite configurations in the other half of the molecules. These meso isomers have no optical activity, but they do contain chiral centers.
OK, OK, I confess, the previous two paragraphs are intended to outline a lot of jargon. This jargon is necessary though. Chiral molecules are ubiquitous in organic chemistry: it would be much harder to talk about stereochemistry (there I go again) if these terms had not been coined. If organic chemists need them, so do you to pass introductory organic chemistry. They are fundamental and come up many times in the course. Students have mastered these concepts if they can explain them to a newcomer to the area (sometimes called The Feynman Method in the psychology of learning).