If you are a high school student, your teacher may mean something like this: sometimes it is not convenient to measure the mass and volume of a substance directly, but it is better to measure the mass change and volume change of a substance. For example, measure the density of liquid, pour a little liquid at a time, measure the change of its mass and volume at a time, calculate the measurement results at each time, and then take the average value to get a more accurate density value.

If you are a college student (or have studied calculus), then your teacher may mean this. Most substances are actually heterogeneous. In order to get an accurate density value, it is scientific to measure the mass dM and volume dV of trace elements from a microscopic point of view, that is, P=dM/dV= triangle M/ triangle V (D stands for differential).