Pixel size in µm (micrometers) is the physical size of one photosite on the sensor. For example, 1 µm is one millionth of a meter.

It is not the same as sensor size. Sensor size describes the overall dimensions of the sensor, while pixel size describes how large each individual pixel is. If you know both, you can roughly infer how many pixels fit on the sensor.

Smaller pixels let more pixels fit into the same area, which can increase potential resolution and fine detail. The tradeoff is often that very small pixels can be associated with more noise. However, pixel size alone does not determine image quality.

In general, for sensors of similar technology, a larger sensor tends to offer advantages such as lower noise and better dynamic range, but “bigger is better” is not always true for every use. For distant subjects like wildlife or the moon, a smaller sensor with a higher pixel density can be useful because it gives more detail on the subject without needing extremely long lenses.

So: pixel size tells you about pixel density and potential resolution, while overall image quality depends on the whole system: sensor size, resolution, sensor technology, and intended use.