This is the smallest change in the quantity being measured (input) of a measuring instrument that gives a perceptible change in the reading.
This is an important consideration in microscopy. A high resolution image can be magnified to show fine details more clearly. Magnifying a low resolution image shows a larger version of the same blurred details.
Be careful not to confuse accuracy, precision and resolution. Accuracy is how close to the true value a reading is; precision is a measure of reproducibility – how close together your results are – how much random variation they have a resolution is the smallest change detectable with your equipment.