1. Density
How much stuff

2. Mass per cubic metre
The basis for all comparisons in density is water:
Water has a density of 1000kg per cubic metre (kg/m3)
Or if you like:
1 gm/cm3
Or just simply 1

3. Solids liquids and gases
The density of solids and liquids is easy because the particles are touching and for the most part cannot be compressed.
But… the density of a gas depends on how compressed it is.
The least dense material we know would probably be the tail of a comet - you could lift a cubic kilometre of it and it would have a mass of about 1 kg.
Try lifting a cubic kilometre of water, or osmium (a piece of osmium the size of a laptop would weigh about the same as a large bucket of water - think lead on steroids), or anything come to that!!!!

4. Finding density
Density needs two measurements:
Its mass.
Its volume

5. Working out density
Note the level of water in a measuring cylinder, then completely submerge the object under study. (Thank Archimedes for this little gem).
This will give you its displacement - its volume.
Then we need its mass.
Mass/volume = density
If it is greater than 1 gm/cm3 or 1000kg/m3 then it will sink less than this value it will float.