A measure of how much matter is in a certain volume.
This gold bar is quite small but has a mass of 1 kilogram (about 2.2 pounds), so it contains more matter than a similar sized piece of wood.
So gold is more dense than wood.
The density of water is about 1 kg per liter (a liter of water is about 1 kg), so anything that floats has a lower density, and anything that sinks is more dense.
Density also has a general meaning of "how much per single unit". So "the ground is dense with weeds" means there are a lot of weeds per square meter (or other unit measure).
