Krippendorff (2012, p.24) describes it thus: “Content analysis is a research technique for making replicable and valid inferences from texts (or other meaningful matter) to the contexts of their use“. The nature of a research design using this technique is shown in the figure below (source: Krippendorff, 2012, p.83). The framework is simple and general, employing only a few conceptual components: 
1. A body of text, the data that a content analyst has available to begin an analytical effort .
2. A research question that the analyst seeks to answer by examining the body of text.
3. A context of the analyst’s choice within which to make sense of the body of text.
4. An analytical construct that operationalizes what the analyst knows about the context of the body of text.
5. Inferences that are intended to answer the research question, which constitute the basic accomplishment of the content analysis.
6. Validating evidence, which is the ultimate justification of the content analysis.
He goes on to explain what this means.
“As a technique, content analysis involves specialized procedures. It is learnable and divorceable from the personal authority of the researcher. As a research technique, content analysis provides new insights, increases a researcher’s understanding of particular phenomena, or informs practical actions. Content analysis is a scientific tool.
Techniques are expected to be reliable. More specifically, research techniques should result in findings that are replicable. That is, researchers working at different points in time and perhaps under different circumstances should get the same results when applying the same technique to the same phenomena. Replicability is the most important form of reliability.
Scientific research must also yield valid results, in the sense that the research effort is open to careful scrutiny and the resulting claims can be upheld in the face of independently available evidence. The methodological requirements of reliability and validity are not unique to but make particular demands on content analysis.
The reference to text in the above definition is not intended to restrict content analysis to written material. The phrase “or other meaningful matter” is included in parentheses to indicate that in content analysis works of art, images, maps, sounds, signs, symbols, and even numerical records may be included as data – that is, they may be considered as texts-provided they speak to someone about phenomena outside of what can be sensed or observed. The crucial distinction between text and what other research methods take as their starting point is that a text means something to someone, it is produced by someone to have meanings for someone else, and these meanings therefore must not be ignored and must not violate why the text exists in the first place. Text – the reading of text, the use of text within a social context, and the analysis of text – serves as a convenient metaphor in content analysis.“
Reference:
Krippendorff, Klaus. Content analysis: An introduction to its methodology. Sage, 2012.