In part 3 of the book Geertz defines religion as "a cultural system", calling for a wider approach to the study of religion that incorporates modern theory alongside the works of Durkheim, Veber, Freud and Malinowski. By cultural system Geertz means 

an historically transmitted pattern of meanings embodied in symbols,a system of inherited conceptions expressed in symbolic forms by means of which men communicate, perpetuate, and develop their knowledge about and attitudes toward life. (p. 89)

The full definition that Geertz gives of religion is:

(1) a system of symbols which acts to (2) establish powerful, pervasive, and long-lasting moods and motivations in men by (3) formulating conceptions of a general order of existence and (4) clothing these conceptions with such an aura of factuality that (5) the moods and motivations seem uniquely realistic. (p. 90, numbers within the text are Geertz's) 

What Geertz means with his definition is that, through symbols, religion argues that we should behave in certain ways and not others because of the way the world is. Religious symbols lead  believers to behave according to certain values, convincing them that there is a superior order in spite of the chaos of everyday existence and seeing them through the personal crises. Thus religion and its symbols give the values they represent a concrete application in our daily reality.