Mass is the amount of matter that an object contains [Matter refers to the material an object is made of]. In other words, mass is a measure of how much “stuff” is in an object. Mass is usually measured in grams or kilograms. For example, a big rock has more mass than a small pebble. The mass of an object can also be changed if more matter is added to it or if matter is removed from it.
Very Simple Definition:
Mass is a measure of how much stuff is in something. For example, a truck has more mass than a bicycle because it has more stuff in it. We can measure mass using units like grams or kilograms.