What is the unit for mass?

Mass is a concept that is difficult to describe without referring to its own definition; in many places, you'll find a circular statement that refers to "matter." A more physics-based concept involves the distinctions between matter, mass, and weight. If you're getting into physics, this is important, but for a biology or chemistry class, where you have a constant amount of gravity and you're really only talking about things taking place on Earth, you can usually treat weight and mass as the same thing.
The international unit of mass is the gram, which is approximately the weight of a drop of water. Grams are a good unit of measure for chemistry experiments, but for large objects like people or cars, or for large-scale reactions like industry or physics calculations, much larger units such as kilograms need to be used.
https://www.nist.gov/