A material object spans certain distances. In other words, it takes up space. This property is called volume.
Volumen is a magnitude which is measured in m³ in the International System of Units.
Mass is related to the amount of matter that is contained within a given volume. Mass is a magnitude which is measured in kg in the International System of Units.
The ratio of mass to volume is called density.
Density is a magnitude which is measured in kg/m³ in the International System of Units.

density = mass / volume

You can practise calculating densities here.
The following visual will allow you to learn how to calculate densities.