The term theory-theory is a theory

Theory

The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...

 in cognitive development

Cognitive development

Cognitive development is a field of study in neuroscience and psychology focusing on a child's development in terms of information processing, conceptual resources, perceptual skill, language learning, and other aspects of brain development and cognitive psychology compared to an adult's point of...

 that children construct theories to explain everything they experience.

According to theory-theory, the best idea and explanation of mental processes in young children is that humans "always seek reasons, causes, and underlying principles" for what they experience. The essential idea of theory-theory is that children do not want simple logical definitions but, rather, seek fuller explanations of various things, especially of those that involve them. The term originated in the 20th century, and the concept is also referred to as model theory.
Theory-theory differs from the Theory of Mind

Theory of mind

Theory of mind is the ability to attribute mental states—beliefs, intents, desires, pretending, knowledge, etc.—to oneself and others and to understand that others have beliefs, desires and intentions that are different from one's own...

(which concerns mental states of people) in that the full scope of theory-theory also concerns mechanical devices or other objects, beyond just thinking about people and their viewpoints.