updated: 2023-02-09
What do we mean by "understand" in science? Qualitatively, we understand something if we are able to explain how it works and predict its quantitative consequences in concrete situations.
Lord Kelvin argued that scientific understanding is connected to the ability to build a causal model. He put:
“It seems to me that the test of ‘Do we or not understand a particular subject in physics?’ is, ‘Can we make a mechanical model of it?’"
In other words, scientific understanding is only achieved by devising a mechanical model (see Kelvin's dictum revived: The intelligibility of mechanisms and On scientific understanding with artificial intelligence | Nature Reviews Physics)
However, the inability to devise a mechanical model does not imply a lack of scientific understanding. Indeed, mechanical modeling is not universally applicable, for example, in quantum physics, although the theory provides qualitative explanations and predictions.
Scientific discoveries and understanding are, although they are often entangled, distinct in the sense that discoveries are possible without understanding. Indeed, many scientific discoveries predate scientific understanding. Examples include planetary motion, superconductivity, and cosmological microwave background.