The mass is defined as the quantity of matter that a body could have. It is different from the weight, because the weight is a force itself. Usually at the SI base unit, the mass is measured in kilograms, which is denoted by "kg".
Now, the density is defined as the quantity of mass per unit volume. So in math therms it tells you how many mass could you have in a defined unit volume, in other words
where m denotes the mass of the object and v the unit volume. So, for example, you could find that the density of the water is
then it is saying you that if you fill with water a cubic whose sides are each one of 1 meter, then it will have a mass of 997kg.
Remember that given the mass and the volume of an object to get its density, you only need to divide the mass by the unit volume.