The statement you mentioned is commonly associated with the philosophical interpretation of quantum mechanics known as the "Copenhagen interpretation." According to this interpretation, there is no element of physical reality, such as definite properties or states, until they are measured or observed.
The Copenhagen interpretation, formulated by Niels Bohr and Werner Heisenberg among others, emphasizes the role of measurement in quantum mechanics. It suggests that the act of measurement or observation "collapses" the quantum wavefunction, which represents the range of possible states of a quantum system, into a specific outcome. Prior to measurement, the system exists in a superposition of all possible states simultaneously.
This interpretation does not imply that reality does not exist until observed but rather suggests that our knowledge or description of reality is constrained by the act of measurement. In other words, until we interact with a quantum system and measure its properties, we cannot assign definite values to those properties. The system is described by probabilities instead.
It's important to note that the Copenhagen interpretation is not the only interpretation of quantum mechanics, and there are alternative interpretations, such as the many-worlds interpretation and the pilot-wave theory, that propose different explanations for the nature of reality and the role of measurement in quantum mechanics.
Ultimately, the interpretation of quantum mechanics remains a subject of ongoing debate among physicists and philosophers, and the question of the nature of reality at the quantum level is still an open and complex topic.