If the topic you are about to discuss requires the use of several vocabulary words, review them and give an example of each. If you are going to use previously learned but unique symbology, review the symbols and their meanings. At the beginning of each class, briefly review the content of the previous class, simply to reload it in your students' minds. If you are going to present a new idea or technique, find the three or four important concepts that are required to understand the new idea, and review those concepts. These three or four background concepts set up a context for the new material.
Review shows that the new material is a logical extension of previous knowledge.
Review is important because human beings cannot remember everything they have learned instantaneously, especially if the knowledge is a year or two old. If they don't recall or understand a key concept or technique, then everything you say in class is meaningless, and will not be understood. Repetition and Examples People learn through repetition. No idea or fact is completely understood the first time it is seen. For complete understanding, the idea should be shown in a variety of contexts and applications, and should be connected to as many previously learned concepts as possible.