The short answer to this question can be found in the title of the collection of essays being referenced, The Interpretation of Culture. Geertz argued that culture was basically like an idiom, or a language, within which human actions could make sense. Thus human actions or behaviors were signs and symbols that could be understood in the context of a specific culture. But they could also be used to understand the culture itself. Semiotics refers to the interpretation of signs and symbols, and in the introduction to The Interpretation of Culture, he writes explicitly that his view of culture is semiotic:

The concept of culture I espouse . . . is a semiotic one. Believing, with Max Weber, that man is an animal suspended in webs of significance he himself has spun, I take culture to be one of those webs, and the analysis of it to be therefore not an experimental science in search of law, but an interpretive one in search of meaning. It is explication I am after, construing social expressions on their surface enigmatical.

In practice, Geertz referred to his semiotic approach to anthropology as "thick description." He looked at every aspect of a particular behavior that made no sense to him, as an outsider to a particular culture, and tried to use it to untangle the "webs of significance" in which it was located. In other words, he tried to "translate" the actions that he understood as symbolic in order to understand the larger culture at work. Geertz's most famous exercise in semiotic "thick description" is his analysis of a cockfight in Bali, described in chapter 15 of The Interpretation of Culture. These cockfights were attended exclusively by Balinese men, who gambled extravagantly on the results of the fights. They often wagered far more than they actually could afford if they lost. Geertz interprets this "deep play" in light of Balinese concepts of masculinity and prestige, concluding that in the Balinese cock fighting rings, "it is only apparently cocks that are fighting there. Actually, it is men." In short, he looks at a particular set of behaviors that are foreign to him and tries to make sense of them by looking at the overall culture, or "language" within which they are expressed.

The Interpretation of Cultures by Clifford Geertz is a collection of essays that shows the evolving pattern of Geertz's thought rather than a singular work with a unified theoretical stance. Nonetheless, the essays contained in it exemplify what might be described as an essentially structuralist approach to anthropology, in which culture is seen as an enabling structure for human interactions.

When Geertz uses the term semiotic or interpretive, he suggests that we can "read" a culture as we read a language in terms of a background of rules ("langue") and specific cases or statements ("parole") and that the job of anthropology as a discipline to map the underlying grammar or syntax of culture in order to then be able to interpret specific cultural phenomena.