Chiral molecules have a point of asymmetry - a chiral center. Typically a carbon with four different groups attached to it is a chiral center. Know the rules for determining R or S configuration. Diastereomers have more than one chiral center per molecule - and some of these chiral centers are the same, others are different. Meso compounds have more than one chiral center, but the molecule as a whole is NOT chiral because the molecule itself is symmetrical - it has a point or plane of symmetry.
Optical activity is a property of chiral compounds.
Chapter 7 introduces the Nucleophilic Substitution reaction and its mechanisms, and returns to the Elimination reaction which was introduced with Alkenes. Be sure to review nomenclature and know how alkyl halides fit into the picture. The leaving group is typically a halide ion, or a protonated oxygen. Hydroxide is NOT a good leaving group, and given that H-OH is not a strong acid this is not surprising. Another complication is that Elimination is ofter a competing reaction.